gtk3x: Replace GDK_MOD1_MASK with GDK_ALT_MASK
gtk-3.98.3 renames GDK_MOD1_MASK as GDK_ALT_MASK. Change our gtk3x-client code to match.
gtk-3.98.3 renames GDK_MOD1_MASK as GDK_ALT_MASK. Change our gtk3x-client code to match.