Add ability to move selected item on top (fixes #683)

[goggle]

As described in #683 this PR adds a new navigation feature to RTV, that allows to move the currently selected item on top of the visible window.

[michael-lazar]

Thanks for the PR (and test!). I want to sit on it for a little while to ponder about the key binding, but overall your code additions look pretty clean and straightforward.

[goggle]

Yes, I wasn't sure about the key binding either. It would be nice if it is something on the right side of the keyboard, since the other navigation keys are also on the right side.
There might also be a better name than focus_up.

[jparkrr]

This is a dope feature. Don't know about the keybinding, but would love to see it merged.

[goggle]

Yeah, on a Swiss QWERTZ keyboard to - key is really convenient to use for this, but probably not on an American QWERTY keyboard. I think the corresponding key on an American keyboard would be /. But this key is already reserved for the prompt.

Maybe the zero key 0 would be a good alternative?