- fix unittest assuming the double-insertion of annotation
  throwing an IllegalArgumentException was intentional
- remove SwModify/SwClient in SwXTextField
- also: add basic C++ api test

Change-Id: Ia4657dc65dfadc3e975bdf409bd5e43413ea1f5a
Reviewed-on: https://gerrit.libreoffice.org/71452
Tested-by: Jenkins
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@libreoffice.org>