Related fdo#81475 Improve toolbar tooltips in Writer standard toolbar

and show the shortcut in the tooltip also when there is a custom tooltip

Change-Id: Ia3f5d17df3769479f7aacf63a83507ccb8593103
Reviewed-on: https://gerrit.libreoffice.org/11303
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>