Use weakrefs in add_event_handler?

[almarklein]

By registering callbacks via add_event_handler, references to objects are stored and can prevent objects from being cleaned up. Is this behavior that we want, or should we use weakrefs?

[almarklein]

Some options:

1. Store the given callback with a weakref. This would break e.g. lambdas and closures, so nah.
2. Store the callback, but (only) use a weakref if its a method. This could kinda work, but it can also feel inconsistent, especially since a closure can also hold a reference to an object.
3. Just store all callbacks by reference. If this is a problem, the caller can use weakrefs in a way that works for the use-case at hand.

I'm leaning hard towards option 3. Created #398 to "implement" that.

[Korijn]

I support that preference for option 3. It doesn't need to be solved here necessarily.