SCM - improve focus movement

I did some improvements in the ActionBar focus which I think reflect positively in the SCM view. However there is still two corner cases which I think we could improve:

1. Focus onto an element in Changes view, press tab -> focus jump to inline actions (great)
2. Press shit + tab -> focus is moved to the Message box 🐛

It would be cool if tab and shift+tab are always paired, thus shift+tab after a tab would bring you back to the original item.

One more corner case

1. Focus onto the first item in changes
2. Press shift + tab -> focus is moved in title bar actions 🐛
3. I would expect the focus to move into the commit box

Overall I think the focus navigation in SCM works nice, thanks to this "top level" up and down navigation which allows to focus into something.

fyi @joaomoreno