LibGUI: Add ModelClient abstract class and allow registering clients

This solves a problem where the SortingProxyModel doesn't
receive the on_update call because other code overwrote
the handler later on.

Libraries/LibGUI/Forward.h

@@ -61,6 +61,7 @@ class Painter;
 class ResizeCorner;
 class ScrollBar;
 class Slider;
+class SortingProxyModel;
 class SpinBox;
 class Splitter;
 class StackWidget;