feat(weakref): support weakref.WeakMethod in state.change and ctrl

[finetjul]

It's important to use WeakMethod in order to not have the method callbacks hold a reference onto the object and let the object be destroyed.

This does not work with decorators though...

[finetjul]

@jourdain shouldn't the decorators @state.change("...") and @controller.add("...") use weak refs on the decorated methods ?

[jourdain]

Not necessary but we could add @state.change_weak(...) and @controller.add_weak(...)

[finetjul]

@jourdain can you please merge, I'll do a separate PR for the decorators