Use RevisionUnion in QNameModule

QNameModule holds an optional Revision, which is inconvenient. This
patch retrofits RevisionUnion into it.

Since we are in this area, improve instantiation api, introducing of()
and ofRevision() convenience methods and deprecate create() methods.

Since QNameModule is a WritableObject composed of XMLNamespace and
RevisionUnion, make the two also WritableObjects and delegate to them.

Change-Id: Ib5349ef324d73340b285e9634e579825e55ab95d
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>