RowSelection::HandleMouseDown: always forward to the SelectionEngine,
even if the hit row is already selected.
This ensures proper de-selection handling in case of Ctrl+Click

Change-Id: I46fd63332101fd078c6341c94575dc69f48f8a68
Reviewed-on: https://gerrit.libreoffice.org/549Reviewed-by: Miklos Vajna <vmiklos@suse.cz>
Tested-by: Miklos Vajna <vmiklos@suse.cz>