LibGUI: Implement persistent indices for models

This patch adds persistent indices to models.  A PersistentModelIndex is
a ModelIndex that will survive most model updates (provided that the
data the PersistentModelIndex points to has not been removed by the
model's data store).  PersistentModelIndex objects can be safely held
by objects outside the model they originated from.

Userland/Libraries/LibGUI/Forward.h

@@ -49,6 +49,8 @@ class MultiView;
 class OpacitySlider;
 class PaintEvent;
 class Painter;
+class PersistentHandle;
+class PersistentModelIndex;
 class RadioButton;
 class ResizeCorner;
 class ResizeEvent;