[Pharo-project] extracted the Cmd-q autocompletion to it's own class

So it can play with the same mechanism as O/ECompletion does. Attached is a changeset with a little class. I'm not sure how to package it yet, but I think it has to be packaged along with SelectorChooserMorph and others. It's related to this issue: http://code.google.com/p/pharo/issues/detail?id=439&q=cmd%20q&colspec=ID%20T... Guille