Standardize control flow between engine subsystems

We could improve the way individual subsystems in the engine (presenter, renderer, gamestate, networking, ...) talk to each other by using a standardized method for all communications. For this to work, we would need to define how data should be sent, fetched or manipulated during transfer and find a flexible solution that is reusable for all subsystems.

The main benefit of this would be less maintanance (since we don't need to figure out a new solution for every subsystems) and better code readability. It should also improve thread safety as the subsystems usually run in their own threads and communication between them form a critical path. We also want to minimize interdependencies between subsystems as much as possible.

https://github.com/SFTtech/openage/pull/1492 and https://github.com/SFTtech/openage/pull/1497 already address parts of this issue by handling communication in dedicated objects and using a one-way control flow.

We can use other implementations as a reference, e.g. https://developer.android.com/kotlin/flow

Start by reviewing pull requests 1492 and 1497, then trace how the presenter, renderer, gamestate, and networking subsystems currently communicate. Define a reusable communication approach covering sending, fetching, manipulation, thread safety, and reduced interdependencies; done means the approach is agreed and applied consistently across the relevant subsystems.