Focus current theme/language in palette

[alichay]

Would close #642

The implementation here is a little bit weird, but this is less trivial than it seems

• If you just set the index in PaletteViewData::run, it gets overridden on the next widget update cycle
• If you fix that, you still need to have the widget auto-scroll to the selected item. The usual scroll code only happens on update, not creation, and it only fires if the old index != the new index, so this also requires special-casing for when there's a default selection.

If someone has any pointers on how this could be cleaned up or made more idiomatic to the codebase (first contributions are always tricky!) I'd love to hear it. Thanks!

(especially this change to lapce-ui/palette.rs where Druid doesn't implement these shared methods with a trait, so I had to make the function generic over a closure that forwards to the actual method. it's pretty nasty)