GtkAction: Hold a reference to proxy widgets

GtkAction: Hold a reference to proxy widgets

Previously we expect that a proxy widget holds a reference to the
action via gtk_activatable_do_set_related_action().  However, it
is possible for the widget to still be in a floating state when
it adds itself to the the action.

This is a problem when gtk_action_get_proxies() gets called, because
we return floating objects back to the user.  And language bindings
aren't going to be expecting that.

Fix this by calling ref_sink() and unref().

https://bugzilla.gnome.org/show_bug.cgi?id=657367