Make BindableProperty add to bindable_properties

[nathanjshaffer]

Make BindableProperty add to bindable_properties during binding instead of waiting till next value change. This fixes a potential issue where a large quantity of BindablePropertys were bound at once would cause a large resource usage if the __set__() method was not called immediately after binding

based on discussion #4101

[falkoschindler]

Thanks a lot for this pull request, @nathanjshaffer!

Can you, please, share a reproduction for the specific problem of "large resource usage" when binding many bindable properties at once? I'm still not sure how this might happen.

Your example in #4101 (comment) was missing an initializer def __init__(self) that creates the attribute, e.g., self.bar = ''. This is why the binding couldn't make use of the bindable property.