Push/Pop-ing the cursor led to selection loss.
SelectHiddenRange() immediately returns false if the current cursor
hasMark(), so avoid all of the bugs and expensive push/pop routines
when there is a mark already.

Change-Id: I4624a3e0b2267942812d0429d527ad97962ec7fc
Reviewed-on: https://gerrit.libreoffice.org/19108Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>