Panel handling cleanup

See commit message.

This splits up panel state handling from hints and from docked mode.

At the same time, the panel handling was moved to Rust to have a real state machine.

This has more code than before, because having behaviour encoded as a state machine has shown plainly how many corner cases we have. I think they might even be actual complexity we just kind of got away with before (e.g. what to do with two consecutive size change requests? Follow the trail of long comment blocks). Some of the new complexity comes from freshly added output awareness.

Draft because it's based on !528 (merged) Draft because the original base was broken. Waiting for !535 (merged)