Add overrides for Gtk.Button set/get_focus_on_click
Add overrides for Gtk.Button set/get_focus_on_click

Fixes #371. Gtk.Button.set_focus_on_click and Gtk.Button.get_focus_on_click
are deprecated since version 3.20. This override calls the Gtk.Widget methods
so that users don't get a deprecation message.

Signed-off-by: Dan Yeaw <2591e5f46f28d303f9dc027d475a5c60d8dea17a@yeaw.me>

GNOME/pygobject,GNOME/pygobject,GNOME/pygobject