Modify some clunky signals to use an autoload singleton event bus instead

[bdunko]

https://www.gdquest.com/tutorial/godot/design-patterns/event-bus-singleton/

The stuff related to a battle starting and to area transitions would probably make sense here, so we don't have to propagate so many signals around.

[bdunko]

Looked at it a bit more and don't think this is helpful.

[bdunko]

We probably do need this actually...

[bdunko]

Made it gooder.