A beginning to the work required for overviews

Hey @sandytrinh,

I had to build this last night for something I was working on and thought it might make a good starting point for the overview work you're going to get started on? Hopefully you haven't yet and can take over from here.

This PR adds the ability to define 'overview actions', that is action buttons that appear on the overview screen. I've made these different to the 'grid actions' so people can have more control.

cheers,
Scott.