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/EllipseTool.cpp

@@ -75,6 +75,7 @@ void EllipseTool::on_mouseup(Layer& layer, GUI::MouseEvent& event, GUI::MouseEve
         draw_using(painter, Gfx::IntRect::from_two_points(m_ellipse_start_position, m_ellipse_end_position));
         m_drawing_button = GUI::MouseButton::None;
         m_editor->update();
+        m_editor->did_complete_action();
     }
 }