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

In https://code.launchpad.net/~afrantzis/mir/multimonitor-examples/+merge/178039 (not merged yet) I have added a base mir::examples::ServerConfiguration which all the demo servers and standalone programs use. We could put the filters there. In order to allow derived configuration classes to extend the filter list, we need to change the filter list interface to use a vector, or other manipulable object, instead of an initializer list.