For a number of implementations (issue 70309 and issue 81480, at least), it is
necessary to allow virtual objects on form controls.

That is, our drawing layer features virtual objects (SdrVirtObj), which refer to
another ("real") object, and behave nearly exactly as this other object. The
advantage of such virtual objects is that you can have multiple of them, all
referring to the same master object. So, to the user, this looks as if the same
object (the master object) appears in several different places in a document.

Now while this works fine for almost all object types in our drawing layer,
there are several problems when it comes to objects carrying an UNO control -
that is, SdrUnoObj instances in general, and form controls in particular.
Virtual objects on UNO objects are simply not supported at the moment, as UNO
objects are treated special in a lot of places in the drawing layer.

This task is to allow virtual objects referring to UNO objects, this way laying
the (well, one) foundation for fixing issue 81480 and issue 70309.