Clarify the order of wl_keyboard.modifiers

Clarify the order of wl_keyboard.modifiers

Consider that we set the modifier for the modifier key as well, and that
you have the option to change your layout when both shifts pressed. This
realized by making the shift keys produce 'Group_switch' when you press
the shift key and the shift modifier is already active. So sending modifier
before the key event will result in 'Group_switch' each time you press
shift.

That being said, the order of modifiers should be updated after the
key/enter event, so it'll only affect future keypresses, not the current one.

See: https://lists.x.org/archives/xorg-devel/2014-July/043110.html

Signed-off-by: Kirill Chibisov <contact@kchibisov.com>