Fixes for HidingContainer to support Multi Select widgets.

[atbinf]

Recommend the below changes to HidingContainer._validate() function. This will allow multi select widget support (show/ hide based on multiply selected items). This also has a fix to compare the mapping property keys with the original value rather than the validated value. The validated value has been, in many cases transformed and thus would not match up with the item in the mapping.

                    if isinstance(c, HidingComponentMixin):
            #Support Multiselect List widgets
            #Show widgets based on pre-validated value
                        if isinstance(data[c.id],list):
                            for i,item in enumerate(data[c.id]):
                                show.update(c.mapping.get(value.get(c.id)[i],[]))
                        else:
                            show.update(c.mapping.get(value.get(c.id), []))

[moschlar]

I'm not really experienced with tw2.dynforms itself, so would you mind making this a proper pull request with some test cases for the behavior?