Stuck dialog box when switching modes

Whe changing operation modes the prime-select is called causing a little freeze of the dialog box.
Adding `while Gtk.events_pending():Gtk.main_iteration()` after https://github.com/linuxmint/nvidia-prime-applet/blob/417dfb28c9b5b13b7589225d6b1e04f7ba7e3061/usr/lib/nvidia-prime-applet/nvidia-prime#L193 seems to solve it efficiently.