wl_keyboard: Add modifier event

This event sends the current keyboard modifier/group state from the
compositor to the client, allowing all clients to have a consistent view
of the keyboard state (e.g. current layout, Caps Lock, et al).  It
should be sent after a keyboard enter event, and also immediately after
any key event which changes the modifier state.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>