- introduce sw::BroadcastingModify for migration
  * still receives .SwModify signals
  * broadcasts those to old SwClients and new SvtListeners
  * add tests
- remove SwClient/SwModify for UNO bookmarks

Change-Id: Icefca478472820b93f4bb59e670e59b60eddfe2b
Reviewed-on: https://gerrit.libreoffice.org/65806
Tested-by: Jenkins
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@libreoffice.org>