data: Require GTK+ 3.11 in glade
backend: Work around popover-over-clutter issues

Until GTK+ learns how to render popovers with transparency
on top of native subwindows, we can make the GtkVolumeButton
appear less broken by making it non-transparent.

This still looks buggy with the popover arrow.

Helps https://bugzilla.gnome.org/show_bug.cgi?id=723556