Add observers to Stitch

[sayhisam1]

Immutability simply isn't performant enough for large games. I am re-adding observers and events to Stitch to solve this issue.

Observers should themselves be immutable though (see Rx)

Leaving this up as an issue for discussion on how this should be implemented

[sayhisam1]

Added Signals in commit a319515
These should provide an easy way to interact with Stitch using observables

[sayhisam1]

Thinking of using Quenty's Rx lib (https://github.com/Quenty/NevermoreEngine/tree/main/src/rx) as an example for how this should be implemented