Filter groups are represented with a simple object in the view model, but they are starting to get fairly complex in the analysis they should perform, and there is starting to be a need for widgets to listen to update events (more specifically, mw.rcfilters.ui.FilterGroupWidgets)
We should transform this.group object storage into an array of mw.rcfilters.dm.FilterGroup models that mixin OO.EventEmitter, emit 'update' events, and have mw.rcfilters.ui.FilterGroupWidget listen to its relevant model's update event.