Use gtk_widget_get_preferred_size with GTK3
libmate-panel-applet: Fix missing include and wrong declaration
clock: Do not try to get width/height of non-existing GdkPixbuf

https://git.gnome.org/browse/gnome-panel/commit/applets/clock?id=4729df0cda021c27b76be072023dbee082e49e47
We need to call gtk_window_set_has_resize_grip also with GTK3 now

Not only with Ubuntu/GTK2