LibGUI: Add optional placeholder to TextEditor

This lets you show some disabled text when no text is entered, and the
editor is not focused.

Libraries/LibGUI/TextDocument.cpp

@@ -675,6 +675,11 @@ void TextDocument::remove(const TextRange& unnormalized_range)
     notify_did_change();
 }
 
+bool TextDocument::is_empty() const
+{
+    return line_count() == 1 && line(0).is_empty();
+}
+
 TextRange TextDocument::range_for_entire_line(size_t line_index) const
 {
     if (line_index >= line_count())

Libraries/LibGUI/TextDocument.h

@@ -139,6 +139,8 @@ public:
 
     virtual bool is_code_document() const { return false; }
 
+    bool is_empty() const;
+
 protected:
     explicit TextDocument(Client* client);
 
@@ -179,6 +181,7 @@ public:
     size_t first_non_whitespace_column() const;
     Optional<size_t> last_non_whitespace_column() const;
     bool ends_in_whitespace() const;
+    bool is_empty() const { return length() == 0; }
 
 private:
     // NOTE: This vector is null terminated.

Libraries/LibGUI/TextEditor.cpp

@@ -461,7 +461,12 @@ void TextEditor::paint_event(PaintEvent& event)
 #ifdef DEBUG_TEXTEDITOR
             painter.draw_rect(visual_line_rect, Color::Cyan);
 #endif
-            if (!document().has_spans()) {
+
+            if (!placeholder().is_empty() && document().is_empty() && !is_focused() && line_index == 0) {
+                auto line_rect = visual_line_rect;
+                line_rect.set_width(font().width(placeholder()));
+                painter.draw_text(line_rect, placeholder(), m_text_alignment, palette().color(Gfx::ColorRole::DisabledText));
+            } else if (!document().has_spans()) {
                 // Fast-path for plain text
                 auto color = palette().color(is_enabled() ? foreground_role() : Gfx::ColorRole::DisabledText);
                 if (is_displayonly() && (is_focused() || has_visible_list()))

Libraries/LibGUI/TextEditor.h

@@ -60,6 +60,9 @@ public:
 
     virtual void set_document(TextDocument&);
 
+    const String& placeholder() const { return m_placeholder; }
+    void set_placeholder(const StringView& placeholder) { m_placeholder = placeholder; }
+
     void set_visualize_trailing_whitespace(bool);
     bool visualize_trailing_whitespace() const { return m_visualize_trailing_whitespace; }
 
@@ -301,6 +304,8 @@ private:
 
     RefPtr<TextDocument> m_document;
 
+    String m_placeholder { "" };
+
     template<typename Callback>
     void for_each_visual_line(size_t line_index, Callback) const;