Notify clients on mode_switch()

This patch implements the notification of clients during mode_switch.
As discussed on IRC, clients are notified of mode_switch only when the
"native" mode is changed and activated. That means that if the native
mode is changed and the compositor had activated a temporary mode for
a fullscreen surface, the clients will be notified only when the native
mode is restored.
The scaling factor is treated the same way as modes.