During investigation of issue 105557 it turned out that creating uno References
from SwModify listeners is inherently not thread-safe.

With the unocore locking refactoring work in CWS swunolocking1, the class
SwUnoCallBack was removed, because all it did was allow for this unsafe use of
its registered listeners; SwDoc::pUnoCallBack is now simply a SwModify.

As a consequence, every time a uno object for a ReferenceMark,
DocumentIndexMark, or Footnote is required, a new one will be created.
This is suboptimal: the uno objects for these should register directly at the
corresponding core objects, which should have a weak reference to the uno
object, thus allowing for re-use of existing uno objects in a thread-safe way.

When this is done, SwDoc::pUnoCallBack can be removed completely.