Use xkbcommon for internal key events too

To manage the keyboard state and send key events to clients we use
the xkbcommon library (unless disabled). That means that we currently
have a separation between key events outgoing to clients and key events
being used internally in the compositor, resulting in different keysyms
being produced in the two cases for the same hardware key.
Filter the key events coming from the underlying platform and modify
them according to the xkb state instead.

Change-Id: I4c6678be68e5b214b2b9cd0222dc9f425c580085
Reviewed-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>