Insert options during MergeCells to empty hidden cells
Three options:
- Move contents to first cell (previous Yes)
- Keep contents in covered cells (previous No, default)
- Empty covered cells (new option)

To be done: link to the help system
https://gerrit.libreoffice.org/27467/

Change-Id: I98e85296591cce8ba789d282cead1f1010e5e2ce
Reviewed-on: https://gerrit.libreoffice.org/27463Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Laurent BP <laurent.balland-poirier@laposte.net>