Automatic unregistering of BindableProperty objects

[andybayer]

First draft to fix the issue reported in #4109.

Replaces the values in the binding.bindable_properties data structure, which acts as a "registry" for available bindable properties, with weakref.finalize objects. Previously, there was a permanent reference to the owner of the BindableProperty, which was never cleared unless explicitly removed with binding.remove.

I also added some very basic tests for this behavior. You may need to refactor these slightly.

What I did not test, and what in theory should still be a problem, is when 2 models have bindable properties and one model binds to a value of the other. Then permanent references to the models are kept in binding.bindings, which are never automatically cleaned up.

[falkoschindler]

Thanks a lot for this pull request, @andybayer!

I'm just wondering: Have you tried replacing the Dict with a Set, as mentioned in #4109 (comment)? Wouldn't it avoid the need for weakrefs?

[andybayer]

Yes, I tried replacing the Dict with a Set and everything worked as expected. I started to implement this fix, but suddenly I thought that the Set also needs to be cleaned up, or else deleted items will clutter it up. So I thought an automatic cleanup action was needed.

I added the cleanup action as a new value to the Dict. Actually, I think it's not the best design because the Dict now has two jobs. It serves as a quick lookup of whether an object-attribute-pair is a BindableProperty and also as a storage for abortable cleanup actions. Perhaps we should not store a reference to the cleanup action at all, since it does no harm to run it after the object has already been manually removed with binding.remove?

[andybayer]

So what would you suggest as the next step(s) @falkoschindler?

Shall I change something?