Allow focus_next and focus_previous to be limited to a container

[willmcgugan]

Currently focusing considers the entire DOM. For some use cases where focusing is done manually we may want to limit where the focus lands.

Suggest we add a selector to focus_next and focus_previous to limit the focusable widgets.

[TomJGooding]

Is this not already possible with Screen.focus_next, or have I misunderstood?