Move TabbableContainer focus movement logic into helper function

[minetoblend]

Companion to ppy/osu#35439

Moves the focus movement logic into as an extension function to cover 2 use-cases:

• implementing ITabbableContainer without inheriting from TabbableContainer
• being able to control focus movement from the parent

Also adds the option to navigate to the first valid focus target if no child is currently focused through the requireFocusedChild parameter (naming kinda bad I know, but I couldn't think of anything better).

[minetoblend]

Highlighting this line because it's the only line that doesn't match the original implementation. Should act the same as before unless requireFocusedChild is false.

[smoogipoo]

Couldn't you just put a TabbableContainer in the hierarchy somewhere?

[minetoblend]

@smoogipoo While it's theoretically possible to use that class, I can't extend TabbableContainer in DrawableTernaryButton itself since it already extends from OsuButton so I'd have to put it as an intermediary drawable somewhere and having the focused drawable be another element than the one being interacted with seems like a code smell to me.

Another reason is that I'm hoping to eventually have this logic be handled by the TabbableContentContainer. There's a bunch of caveats with having the logic for this inside TabbableContainer. For exampe, if you navigate to a TabbableContainer without a TabbableContentContainer set you cannot navigate away from it with tab anymore, which is an easy thing to overlook.

[smoogipoo]

I suppose this is fairly harmless in isolation