fdo#64302: Writer’s Special Character dialog could be simpler

Change-Id: Ia321a9df77a8741245378e5b995b3bc6922266e8
Reviewed-on: https://gerrit.libreoffice.org/11621
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>