Discussion: adding event watchers

[SuperRedingBros]

Description

Basically, the goal is to add event watchers as a new potential way of handling events, to complement the standard event loop without breaking compatibility.

Currently, the syntax is like this:

@pygame.event.add_event_watcher
def eventWatcher(event):
    ... # use the event somehow

Whenever events are added to the queue, they are sent to the event watchers, they can still be accessed from an event loop as usual, however.
@ankith26 and @aatle proposed making them behave more like SDL_appEvent, however, IMO that is somewhat more intrusive to the standard event loop. I am curious what other people think about it.

Currently, any exceptions thrown by the watcher are logged and otherwise ignored, this is probably not great. Again, I am interested in other people's input on the matter

I already have opened a PR for this, #3637; there are some other details and an example usage of the feature there as well.

[aatle]

I think the watchers could be made to behave like SDL_AppEvent without disrupting the standard event loop.
The watchers would be invoked first upon events, but the events would still appear on the event queue with pygame.event.get() as normal.

Naming of the decorator is a point of discussion but I think pygame.app_event would be the best choice. However, the entire future main callbacks API would probably need to be decided upon before naming this decorator.

I think it might be nice for watchers to be able to filter by event type.