Code review comment for lp:~robert-ancell/mir/alt-ctrl-backspace

Now updated with Alexandros' changes so the filter can be shared between all the examples.

I don't think this should be in mir::DefaultServerConfiguration since we don't want a magic key combination to kill Mir. For "real" Mir servers you should VT switch and kill the server. I'll make a separate MP to add VT switching key combinations by default.