Add isSelectedDate to dijit._Calendar

[ryangroth5]

There is a basic assumption that the _Calendar has one selected date which limits the ability to select multiple dates on the _Calendar. While this might be an uncommon case for the dijit.* components, it is an requirement to facilitate with the addition of one method.

Please review the following patch and if it doesn't seem to violate any design principles the additional capability that subclasses could provide would be beneficial to the community. I actually think that it improves the readability of the code as well.

I have a CLA on file if you want to incorporate the patch as is.

[ryangroth5]

adds isSelectedDate to _Calendar

[bill]

OK, I'll add it but as _isSelectedDate since it's not a typical extension function (ie, it only makes sense if you are subclassing Calendar, it doesn't make sense to just define that one method).

[bill]

(In [17544]) Make it possible to subclass _Calendar to have multiple selected dates. Fixes #9317 !strict.