SfxUndoManager is the only implementation of the IUnderManager
"interface", and it lives in the same header.
Plus this way we can get rid of some covariant parameters,
which will make using std::unique_ptr easier.

Change-Id: I6661b9876b18da830bead78794d237886cd3c5c9
Reviewed-on: https://gerrit.libreoffice.org/57317
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>