Rethink focus

[robinp]

Now focus is ad-hoc: needs initial click for viewer pane to get focus, and after that we do best effort to give back focus after operations in different panes (like after click in filetree or refs).

We should maybe give visual indication of where the focus is? Or make key combinations work regardless of where the focus is - for example, a boundary-respecting mouse-pointer search could work regardless of focus, as long the pointer is at a sensible place.

Sounds like mouse events should work globally, while key-only events (we don't have such now) would need to take focus in account.