Arrow-key navigation in UITreeContainer

Alright, so I think I got this thing in a good enough state now. I've tested this only on Polycode's project browser so far. Besides navigating trees with the arrow keys, here's a few more supplementary things:

* added `scrollToNode()` method to `UITreeContainer`, which is how the container scrolls via arrow keys. This method will hopefully prove useful elsewhere if someone wants to find and show a specific node for some reason
* added some getters to `UITree` and `UIScrollContainer` to be able to perform necessary calculations for scrolling and nav
* made UITreeContainers focusable, but there's no visual indication of this state either way yet