MultipleChoiceInput with variables choices

[davidebazzi]

I have the requirement to filter out some of the initial possible choices. This is not yet possible neither with the values from the resources neither with the directValues these values are used during BuildShowProperty and can't be changed during the action use. To avoid having to implement from scratch an input similar to MultipleChoiceInput, I suggest to allow to instantiate a derived MultipleChoiceProperty in a derived MultipleChoiceInput. Then in the derived MultipleChoiceProperty it is easy to change the possible choices through the Choices property