Trying to fix the singleton-ness in the plugin manager because it
prevents injecting settings until the game is fully committed (new game
wizard completed). Added the docs describing what I think I've been able
to discover.
* Move the UI code out of the plugin logic.
* Add types where needed.
* Move into game package.
* Improve error handling.
* Simplify settings behavior.
* Don't load disabled plugins.
* Remove knowledge of non-base plugins from game generation.
Fixes https://github.com/Khopa/dcs_liberation/issues/311
