Some GtkSettings property are registered by other classes. This leads
to the "interesting" issue that setting GtkSettings:gtk-button-images
requires that the GtkButton class is referenced first - or that a
GtkButton is created.
https://bugzilla.gnome.org/show_bug.cgi?id=632538
This commit is a cherry-pick from master which (sadly) fixed the issue
only after we started the 2.9x work, and I completely forgot to backport
it. This is needed because GTK 2.x started tripping the warning added in
GObject by this bug:
https://bugzilla.gnome.org/show_bug.cgi?id=698614
Cherry-picked-from: 9f9edb662e
Signed-off-by: Emmanuele Bassi <ebassi@gnome.org>