Support changing the keymap

This adds new API, but does not add a way for the user to set the
keymap. That should be implemented in the compositors.

Change-Id: I28ee7d372f8f578c0d7efb9223cabce49504be1f
Reviewed-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>