[PATCH xwayland] Handle wayland keymap events

[Rui Matos]

These patches add support for changing the X keymap on wayland keymap
events.

I wonder what we should do for X client requests to change the keymap
though. We could:

a) somehow try to change the wayland keymap accordingly, possibly
   through some private compositor protocol;

b) ignore such requests, which seems legal according to both the core
   X protocol and XKB;

c) keep the current status quo, i.e. if an X client changes the keymap
   we have a different keymap for X clients and wayland clients until
   the next time the wayland keymap is updated.

Rui