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.h

@@ -52,7 +52,7 @@ private:
 
     virtual void image_did_add_layer(size_t) override;
     virtual void image_did_remove_layer(size_t) override;
-    virtual void image_did_update_layer(size_t);
+    virtual void image_did_modify_layer(size_t);
 
     void relayout_gadgets();