Allow QSelectSection to work with subclasses of QSelectItemElement.

Changed the ```isMemberOfClass:``` call in the ```addElement:``` method
to ```isKindOfClass:``` to allow for proper subclassing of
QSelectItemElement.

I ran into this bug wondering why my ```AttributedSelectItemElement```
subclass of ```QSelectItemElement```
subclass didn't seem to work as expected.
Merge pull request #527 from bartvandendriessche/enhancement/allow_qselectsection_to_work_with_qselectitemelement_subclass

Allow QSelectSection to work with subclasses of QSelectItemElement.