Invalidate rect cache before moving object

Otherwise I get sw/source/core/layout/anchoredobject.cxx:582:

  <SwAnchoredObject::GetObjRectWithSpaces> - cache for object
  rectangle inclusive spaces marked as valid, but it couldn't be.
  Missing invalidation of cache. Please inform OD.

Change-Id: I3f315b15fca6e2480c11183269be5583e6aea123
Reviewed-on: https://gerrit.libreoffice.org/10977
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Tested-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>