LibGUI: Add gutter indicators to TextEditor :^)

HackStudio's Editor has displayed indicators in its gutter for a long
time, but each required manual code to paint them in the right place
and respond to click events. All indicators on a line would be painted
in the same location. If any other applications wanted to have gutter
indicators, they would also need to manually implement the same code.

This commit adds an API to GUI::TextEditor so it deals with these
indicators. It makes sure that multiple indicators on the same line
each have their own area to paint in, and provides a callback for when
one is clicked.

- `register_gutter_indicator()` should be called early on. It returns a
  `GutterIndicatorID` that is then used by the other methods.
  Indicators on a line are painted from right to left, in the order
  they were registered.
- `add_gutter_indicator()` and `remove_gutter_indicator()` add the
  indicator to the given line.
- `clear_gutter_indicators()` removes a given indicator from every line.
- The `on_gutter_click` callback is called whenever the user clicks on
  the gutter, but *not* on an indicator.

Userland/Libraries/LibGUI/TextEditor.cpp

@@ -290,6 +290,28 @@ void TextEditor::mousedown_event(MouseEvent& event)
         }
     }
 
+    if (gutter_content_rect(text_position.line()).contains(event.position())) {
+        auto& gutter_indicators = m_line_data[text_position.line()]->gutter_indicators;
+        auto indicator_position = 0;
+        for (auto i = 0u; i < m_gutter_indicators.size(); ++i) {
+            if ((gutter_indicators & (1 << i)) == 0)
+                continue;
+
+            if (gutter_indicator_rect(text_position.line(), indicator_position).contains(event.position())) {
+                auto& indicator_data = m_gutter_indicators[i];
+                if (indicator_data.on_click)
+                    indicator_data.on_click(text_position.line(), event.modifiers());
+                return;
+            }
+            indicator_position++;
+        }
+
+        // We didn't click on an indicator
+        if (on_gutter_click)
+            on_gutter_click(text_position.line(), event.modifiers());
+        return;
+    }
+
     if (on_mousedown)
         on_mousedown();
 
@@ -404,7 +426,7 @@ int TextEditor::gutter_width() const
 {
     if (!m_gutter_visible)
         return 0;
-    return line_height(); // square gutter
+    return line_height() * (m_most_gutter_indicators_displayed_on_one_line + 1);
 }
 
 Gfx::IntRect TextEditor::gutter_content_rect(size_t line_index) const
@@ -446,6 +468,18 @@ Gfx::IntRect TextEditor::folding_indicator_rect(size_t line_index) const
     };
 }
 
+Gfx::IntRect TextEditor::gutter_indicator_rect(size_t line_number, int indicator_position) const
+{
+    auto gutter_rect = gutter_content_rect(line_number);
+    auto indicator_size = gutter_rect.height();
+    return Gfx::IntRect {
+        gutter_rect.right() - static_cast<int>(lroundf(indicator_size * (indicator_position + 1.5f))),
+        gutter_rect.top(),
+        indicator_size,
+        indicator_size
+    };
+}
+
 Gfx::IntRect TextEditor::gutter_rect_in_inner_coordinates() const
 {
     return { 0, 0, gutter_width(), widget_inner_rect().height() };
@@ -599,6 +633,28 @@ void TextEditor::paint_event(PaintEvent& event)
         }
     }
 
+    // Draw gutter indicators
+    if (m_gutter_visible) {
+        for (size_t line_index = first_visible_line; line_index <= last_visible_line; ++line_index) {
+            if (!document().line_is_visible(line_index))
+                continue;
+
+            auto& gutter_indicators = m_line_data[line_index]->gutter_indicators;
+            if (gutter_indicators == 0)
+                continue;
+
+            auto indicator_position = 0;
+            for (auto i = 0u; i < m_gutter_indicators.size(); ++i) {
+                if ((gutter_indicators & (1 << i)) == 0)
+                    continue;
+
+                auto rect = gutter_indicator_rect(line_index, indicator_position);
+                m_gutter_indicators[i].draw_indicator(painter, rect, line_index);
+                indicator_position++;
+            }
+        }
+    }
+
     auto horizontal_scrollbar_value = horizontal_scrollbar().value();
     painter.translate(-horizontal_scrollbar_value, -vertical_scrollbar().value());
     if (m_icon && horizontal_scrollbar_value > 0)
@@ -2511,4 +2567,46 @@ void TextEditor::highlighter_did_set_folding_regions(Vector<GUI::TextDocumentFol
     recompute_all_visual_lines();
 }
 
+ErrorOr<TextEditor::GutterIndicatorID> TextEditor::register_gutter_indicator(PaintGutterIndicator draw_indicator, OnGutterIndicatorClick on_click)
+{
+    // We use a u32 to store a line's active gutter indicators, so that's the limit of how many we can have.
+    VERIFY(m_gutter_indicators.size() < 32);
+
+    GutterIndicatorID id = m_gutter_indicators.size();
+    TRY(m_gutter_indicators.try_empend(move(draw_indicator), move(on_click)));
+    return id;
+}
+
+void TextEditor::add_gutter_indicator(GutterIndicatorID id, size_t line)
+{
+    auto& line_indicators = m_line_data[line]->gutter_indicators;
+    if (line_indicators & (1 << id.value()))
+        return;
+    line_indicators |= (1 << id.value());
+
+    // Ensure the gutter is at least wide enough to display all the indicators on this line.
+    if (m_most_gutter_indicators_displayed_on_one_line < m_gutter_indicators.size()) {
+        unsigned indicators_on_line = popcount(line_indicators);
+        if (indicators_on_line > m_most_gutter_indicators_displayed_on_one_line) {
+            m_most_gutter_indicators_displayed_on_one_line = indicators_on_line;
+            update();
+        }
+    }
+
+    update(gutter_content_rect(line));
+}
+
+void TextEditor::remove_gutter_indicator(GutterIndicatorID id, size_t line)
+{
+    m_line_data[line]->gutter_indicators &= ~(1 << id.value());
+    update(gutter_content_rect(line));
+}
+
+void TextEditor::clear_gutter_indicators(GutterIndicatorID id)
+{
+    for (auto line = 0u; line < line_count(); ++line)
+        remove_gutter_indicator(id, line);
+    update();
+}
+
 }