- move DispatchHelper somewhere public
- use it from generic dispatcher call sites in sfx2
- return result of dispatcher calls (conveyed via
  XDispatchResultListener) to calling code, instead of faking it

Change-Id: Ie8041133e99dd99e45819f98798829b96532b9e6
Reviewed-on: https://gerrit.libreoffice.org/24953Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>