RE 6805 Refactor SelectionNotificationService class

Commenting out test, due to link error on windows:
SelectionNotificationServiceTest.obj : error LNK2019:
unresolved external symbol "public: static struct
QMetaObject const
MantidQt::API::SelectionNotificationServiceImpl::staticMetaObject

I don't have access to a windows development system
to fix this.

refs #6805