Move event handling to Rust

WIP because not cleaned up.

Initially intended as a base for some annoying changes needed for the popup, this ended up being a rabbit hole.

The main benefit is handling the lists of pressed/locked keys in Rust.

The secondary immediate benefit is that multiple buttons sharing state will behave correct now.

The tertiary, future benefit is that there's a clear path to implementing a good multi-touch model now (per-touch pressed buttons lists).

Main disadvantage: drawing and state changes are still intertwined, and some naming is confusing :( Needs future work.