PixelPaint: Show more specific Undo/Redo action text

The Undo/Redo actions now tell you what kind of action will be
undone/redone. This is achieved by adding an "action text" field to the
ImageUndoCommand and having everyone who calls did_complete_action()
provide this text.

Userland/Applications/PixelPaint/MainWidget.h

@@ -51,6 +51,8 @@ private:
     ImageEditor& create_new_editor(NonnullRefPtr<Image>);
     void create_image_from_clipboard();
 
+    void image_editor_did_update_undo_stack();
+
     void set_actions_enabled(bool enabled);
 
     virtual void drop_event(GUI::DropEvent&) override;