Reimplement GeditPanel in terms of GtkStack instead of GtkNotebook

The Stack API matches the Panel API better, so basically GeditPanel is
now nothing more than a very thin wrapper around a GtkStack object.

The only drawback is that the bottom panel switcher buttons are ugly,
but it can probably be resolved using some styling.
Implement a new GeditMenuStackSwitcher widget.

This widget is similar to GtkStackSwitcher but uses a dropdown menu. As
of this commit, changing the current page of the stack doesn't work.