PixelPaint: Have layers and images send out notifications on changes

We use this to automatically update the thumbnail in LayerListWidget
when you draw into a layer. We also use it to repaint the ImageEditor
when the image changes somehow. :^)

Applications/PixelPaint/LayerListWidget.cpp

@@ -125,7 +125,7 @@ void LayerListWidget::image_did_remove_layer(size_t layer_index)
     relayout_gadgets();
 }
 
-void LayerListWidget::image_did_update_layer(size_t layer_index)
+void LayerListWidget::image_did_modify_layer(size_t layer_index)
 {
     update(m_gadgets[layer_index].rect);
 }