[PATCH weston] Add a way to update the keymap

[Rui Matos]

We'll need something like this in mutter-wayland to allow people to
add/change their keyboard layouts with gnome-control-center so I
figured I'd start by implementing the basics in weston first.

There's an implementation for a couple of backends and a fix for the
client side which would leak on keymap events.

Comments welcome, especially about the state handling - right now I'm
just reseting the XKB state but I'm not sure this is the correct thing
to do.

Rui