If there is a table cursor, store its point/mark position on the cursor
stack instead of pCurCrsr, which in this case is just one cell in a ring
that contains a SwShellCrsr for every selected cell.

SwCrsrShell::Pop() is surprisingly able to restore the prior state,
because it calls UpdateCrsr(), which detects that the pCurCrsr spans
multiple table cells and re-creates pTblCrsr, and then later on the
first GetCrsr() call SwTableCursor::MakeBoxSels() will restore the ring
of SwShellCrsrs from the pTblCrsr selection.

Change-Id: Ibe9a7f379ece61c7dab1a6f69abaebf42a4c85ec