Rework frozen actors to support tooltips and orders [WIP]

I'm putting this up for discussion and testing, but it needs further work before it can be merged.
It touches a bunch of nasty old fragile code, so i'm expecting to have introduced regressions.

What works:
* Actor tooltips.
* Attack orders.
* Per-player visibility (switching players when observing shows the correct state).

What doesn't:
* Non-attack orders (C4, capture, etc).
* Bibs aren't handled properly when switching players.
* Move order target line still leaks hidden building positions.