When OptionButton is pressed, give focus to selected option

[rburing]

A simple usability improvement: give focus to the currently selected option when an OptionButton is pressed.

The previous behavior was to focus on no option in particular (index -1), which was less clear (find the filled circle), less standard (compare e.g. web browsers) and less convenient. For users of the arrow keys, the previous behavior required (in all but two cases) at least one useless button press (to give focus to the first or last item).

It has been asked on the German Godot Community Discord and in Q&A how to achieve this functionality. As far as I could see it was impossible in plain GDScript. It was easy to implement in C++ (the commit is basically a one-liner), and I see no reason to keep the old behavior or to have any other behavior, hence this humble PR.

[KoBeWi]

Looks ok.

We now have both setter and getter for current index, so maybe it would make sense to expose the setter and make current index a property? (but then the setter needs update() call or something)
Just wondering, the PR is fine as is.

[rburing]

Thanks for the review. I updated the setter to do the visual update(), so now it should also work when called from other places. I held off on exposing a property for now. If someone has a compelling use case for it, I can still do it.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.5.
Edit: Undone, doesn't compile.

[akien-mga]

Actually undid cherry-pick as it relies on APIs which are not present in 3.x. A dedicated backport PR could be made for 3.x if needed.