Made the container for storing multimarks dynamic.
Also let the full row marks to be stored in a dedicated
ScMarkArray object rather than in the multimarks container.

Unit tests for ScMarkData and ScMultiSel will come in a follow up
patch.

Change-Id: I300ff80bebd6f4f39c284c1e8cb7deece82c1bec
Reviewed-on: https://gerrit.libreoffice.org/22163Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>