GtkMenuButton: Submit to action

As an actionable (inherited from GtkButton), a GtkMenuButton
should not set its own sensitivity when it has an action-name
set, but just follow the enabled state of the associated
action.

https://bugzilla.gnome.org/show_bug.cgi?id=738083
Add a note about text view issues

Mention text view background problems and the draw_layer
vfunc in the release notes.