Implement keyboard auto-repeat

Wayland does not send auto-repeat key events, so the clients must implement
it themselves. The times used here are taken from Weston's toy toolkit
implementation.

Change-Id: I13ec7799672ee5ca2904c9b18ac5ec0c8816d6fc
Reviewed-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>
Reviewed-by: Andy Nichols <andy.nichols@digia.com>