PixelPaint: Undo and redo actions

The most used feature of any image editor, undo. Each tool now
notifies the ImageEditor that they completed an action, where
it'll take a snapshot if its current state.

For now, a snapshot is just a copy of the whole image and its
layers. There's a hard limit on the amount of actions it stores.

Applications/PixelPaint/RectangleTool.cpp

@@ -82,6 +82,7 @@ void RectangleTool::on_mouseup(Layer& layer, GUI::MouseEvent& event, GUI::MouseE
         draw_using(painter, rect);
         m_drawing_button = GUI::MouseButton::None;
         layer.did_modify_bitmap(*m_editor->image());
+        m_editor->did_complete_action();
     }
 }