LibGUI: Factor out scrolling logic from GTableView into a GScrollableWidget.

This then becomes the base class for GTableView. I'd like to share as much
code as possible with GTextEditor and any other scrollable widgets.

LibGUI/GScrollableWidget.h

@@ -0,0 +1,38 @@
+#pragma once
+
+#include <LibGUI/GWidget.h>
+
+class GScrollBar;
+
+class GScrollableWidget : public GWidget {
+public:
+    virtual ~GScrollableWidget() override;
+
+    virtual const char* class_name() const override { return "GScrollableWidget"; }
+
+    Size content_size() const { return m_content_size; }
+
+    Rect visible_content_rect() const;
+
+    void scroll_into_view(const Rect&, Orientation);
+
+    GScrollBar& vertical_scrollbar() { return *m_vertical_scrollbar; }
+    const GScrollBar& vertical_scrollbar() const { return *m_vertical_scrollbar; }
+    GScrollBar& horizontal_scrollbar() { return *m_horizontal_scrollbar; }
+    const GScrollBar& horizontal_scrollbar() const { return *m_horizontal_scrollbar; }
+
+protected:
+    explicit GScrollableWidget(GWidget* parent);
+    virtual void resize_event(GResizeEvent&) override;
+    void set_content_size(const Size&);
+    void set_size_occupied_by_fixed_elements(const Size&);
+
+private:
+    void update_scrollbar_ranges();
+
+    GScrollBar* m_vertical_scrollbar { nullptr };
+    GScrollBar* m_horizontal_scrollbar { nullptr };
+    GWidget* m_corner_widget { nullptr };
+    Size m_content_size;
+    Size m_size_occupied_by_fixed_elements;
+};