HackStudio+LibGUI: Implement matching curly brace highlighting

This works for C++ syntax highlighted text documents by caching the C++
token type in a new "arbitrary data" member of GTextDocumentSpan.

When the cursor is placed immediately before a '{' or immediately after
a '}', we highlight both of these brace buddies by changing their
corresponding spans to have a different background color.

..and spans can also now have a custom background color. :^)

Libraries/LibGUI/GTextDocument.h

@@ -15,8 +15,10 @@ class GTextDocumentLine;
 struct GTextDocumentSpan {
     GTextRange range;
     Color color;
+    Optional<Color> background_color;
     bool is_skippable { false };
     const Font* font { nullptr };
+    void* data { nullptr };
 };
 
 class GTextDocument : public RefCounted<GTextDocument> {
@@ -54,6 +56,7 @@ public:
 
     bool has_spans() const { return !m_spans.is_empty(); }
     const Vector<GTextDocumentSpan>& spans() const { return m_spans; }
+    void set_span_at_index(int index, GTextDocumentSpan span) { m_spans[index] = move(span); }
 
     void append_line(NonnullOwnPtr<GTextDocumentLine>);
     void remove_line(int line_index);

Libraries/LibGUI/GTextEditor.cpp

@@ -389,6 +389,7 @@ void GTextEditor::paint_event(GPaintEvent& event)
                 for (int i = 0; i < visual_line_text.length(); ++i) {
                     const Font* font = &this->font();
                     Color color;
+                    Optional<Color> background_color;
                     GTextPosition physical_position(line_index, start_of_visual_line + i);
                     // FIXME: This is *horribly* inefficient.
                     for (auto& span : document().spans()) {
@@ -397,8 +398,11 @@ void GTextEditor::paint_event(GPaintEvent& event)
                         color = span.color;
                         if (span.font)
                             font = span.font;
+                        background_color = span.background_color;
                         break;
                     }
+                    if (background_color.has_value())
+                        painter.fill_rect(character_rect, background_color.value());
                     painter.draw_text(character_rect, visual_line_text.substring_view(i, 1), *font, m_text_alignment, color);
                     character_rect.move_by(advance, 0);
                 }
@@ -1135,6 +1139,7 @@ void GTextEditor::set_cursor(const GTextPosition& a_position)
         update(old_cursor_line_rect);
         update_cursor();
     }
+    cursor_did_change();
     if (on_cursor_change)
         on_cursor_change();
 }

Libraries/LibGUI/GTextEditor.h

@@ -120,6 +120,7 @@ protected:
     virtual void leave_event(CEvent&) override;
     virtual void context_menu_event(GContextMenuEvent&) override;
     virtual void resize_event(GResizeEvent&) override;
+    virtual void cursor_did_change() {}
 
     GTextPosition text_position_at(const Point&) const;