Async event handlers are no longer allowed

[almarklein]

This PR removes support for event handlers that are coroutines (i.e. async functions).

This functionality was initially introduced to allow using async wgpu methods in callbacks. However, in hindsight it does not fit well, and it also prevents handling events inside the native draw event, which causes problem on its own (see e.g. #56).

With pygfx/wgpu-py#739, users have options to handle async methods using callbacks (using promise.then().

So to summarize: disallowing async functions, means that emit() and other methods of the event emitter can now be sync, which means they can be called anywhere, which we need to create a congruent event loop.

(I have more changes to the scheduling planned, to allow asynchronous bitmap presenting, and this change will help in that work.)

Although this is a breaking API change, I doubt that anyone is actually using this coroutines as callbacks.