wayland-keyboard: Send modifiers after the key event

The key event should be interpreted by clients with the modifier state
as it was before the event itself just as in X11 input events.
Achieving this in wayland is a matter of sending the key event first
and the modifiers after (if needed).

This isn't really specified in the wayland protocol but it matches
weston's behavior and should avoid corner cases in clients.

https://bugzilla.gnome.org/show_bug.cgi?id=738238