LibGUI+TextEditor: Make TextDocument modified state track undo stack

This was quite unreliable before. Changes to the undo stack's modified
state are now reflected in the document's modified state, and the
GUI::TextEditor widget has its undo/redo actions updated automatically.

UndoStack is still a bit hard to understand due to the lazy coalescing
of commands, and that's something we should improve upon (e.g with more
explicit, incremental command merging.) But for now, this is a nice
improvement and undo/redo finally behaves in a way that feels natural.

Userland/Libraries/LibGUI/TextEditor.h

@@ -134,6 +134,7 @@ public:
     virtual void redo() { document().redo(); }
 
     Function<void()> on_change;
+    Function<void(bool modified)> on_modified_change;
     Function<void()> on_mousedown;
     Function<void()> on_return_pressed;
     Function<void()> on_escape_pressed;