Add keybindings for switching VTs

Once mutter is started from weston-launch on its own VT, there is
no way to change VT again (for example to actually start an application),
because the keyboard is put in raw mode.
So introduce some keybindings mimicking the standard X ones (Ctrl+Alt+Fn)
that switch the VT manually when activated.

https://bugzilla.gnome.org/show_bug.cgi?id=705861