Begin working on a graphical TextEditor.

It's gonna be a wrapper around a new GTextEditor widget so I can easily
reuse the functionality anywhere I need it. :^)

LibGUI/GTextEditor.h

@@ -0,0 +1,77 @@
+#pragma once
+
+#include <LibGUI/GWidget.h>
+#include <AK/Function.h>
+#include <AK/HashMap.h>
+
+class GScrollBar;
+
+class GTextPosition {
+public:
+    GTextPosition() { }
+    GTextPosition(int line, int column)
+        : m_line(line)
+        , m_column(column)
+    {
+    }
+
+    bool is_valid() const { return m_line >= 0 && m_column >= 0; }
+
+    int line() const { return m_line; }
+    int column() const { return m_column; }
+
+    void set_line(int line) { m_line = line; }
+    void set_column(int column) { m_column = column; }
+
+private:
+    int m_line { -1 };
+    int m_column { -1 };
+};
+
+class GTextEditor : public GWidget {
+public:
+    explicit GTextEditor(GWidget* parent);
+    virtual ~GTextEditor() override;
+
+    void set_text(const String&);
+
+    int content_width() const;
+
+    Rect visible_content_rect() const;
+    void scroll_into_view(const GTextPosition&, Orientation);
+
+    int line_count() const { return m_lines.size(); }
+
+    int padding() const { return 2; }
+
+private:
+    virtual void paint_event(GPaintEvent&) override;
+    virtual void resize_event(GResizeEvent&) override;
+    virtual void mousedown_event(GMouseEvent&) override;
+    virtual void keydown_event(GKeyEvent&) override;
+    virtual bool accepts_focus() const override { return true; }
+
+    void update_scrollbar_ranges();
+    Rect line_content_rect(int item_index) const;
+    Rect cursor_content_rect() const;
+    void update_cursor();
+
+    GScrollBar* m_vertical_scrollbar { nullptr };
+    GScrollBar* m_horizontal_scrollbar { nullptr };
+
+    class Line {
+    public:
+        Line() { }
+
+        String text() const { return m_text; }
+        int length() const { return m_text.length(); }
+        int width(const Font&) const;
+        void set_text(const String&);
+
+    private:
+        String m_text;
+        mutable int m_cached_width { -1 };
+    };
+    Vector<Line> m_lines;
+    GTextPosition m_cursor;
+};