MOF24 — Fix resolution to issue 6905 from MOF2.4 ballot 1

Change the Element/Tag owner composite association to a composite association that symmetrically subsets the Element owner/ownedElement derived composite association from UML.

The resolution to issue 6905 added a property: Tag::owner : Element[0..1]
When EMOF or CMOF would be merged, this property would conflict with UML::Element::/owner : Element[0..1] property (see figure 7.3 in UML2.4 superstructure)
The fix involves the following:

• change the name of ownership property from owner to tagOwner
• symmetrically subset the association end properties of the association UML::A_ownedElement_owner
• make the composite association end, ownedTag, navigable and owned by the association; this is essential to avoid an incompatible API change to UML::Classes::Kernel::Element. However, because navigable owned association ends are not allowed in EMOF, it is necessary to add the ownedTag/tagOwner association in MOF::CMOFExtension instead of MOF::Extension.