Tab focus order

This fixes the tab focus order problem described in #107, and also improves the appearance of focussed buttons so it's more obvious where the focus has gone. It also prevents tab moving focus to the viewer or node graph, but allows them to grab focus on mouse enter as per #119. I tried drawing a border around the viewer/node graph when they have focus, but didn't like the look of it so much - give it a go and if you feel you'd prefer it I'll add one in before we close #119.