This is some naming inconsistency in GTK+ that makes it confusing
to program, (probably left over from naming inconsistency in Xlib).
G_CALLBACK (on_button_press_event), &game);
g_signal_connect (G_OBJECT (window), "button_release_event",
G_CALLBACK (on_button_release_event), &game);
- g_signal_connect (G_OBJECT (window), "button_motion_event",
+ g_signal_connect (G_OBJECT (window), "motion_notify_event",
G_CALLBACK (on_button_motion_event), &game);