This adds gtk_widget_get/set_opacity, as well as a GtkWidget.opacity
property. Additionally it deprectates gtk_window_get/set_opacity and
removes the GtkWindow.opacity property (in preference for the new
identical inherited property from GtkWidget, which should be ABI/API
compat).

The implementation is using the new gdk_window_set_opacity child
window support for windowed widgets, and cairo_push/pop_group()
bracketing in gtk_widget_draw() for non-window widgets.

https://bugzilla.gnome.org/show_bug.cgi?id=687842