Return the kind of cells span from wxGrid::GetCellSize().

Return the kind of cells span from wxGrid::GetCellSize().

Behaviour of GetCellSize() may be very surprising for the unwary as it can
return negative or null "size" of the cell.

Add CellSpan return value to allow the caller to check what kind of cell are
we dealing with easier.

Also document the new return value as well as the function (and matching
SetCellSize()) itself carefully as its behaviour is far from obvious.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63000 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775