vteapp: Unbreak transparency

Mark the window as app-paintable. Otherwise GTK+ will try to optimize
by setting an opaque region for the whole window (gtk+ 3c2c3ab6).

https://bugzilla.gnome.org/show_bug.cgi?id=729884
widget: Update background when opacity is changed

https://bugzilla.gnome.org/show_bug.cgi?id=730023
Post release version bump