Another attempt at 2178. This is a "minimal" patch designed to fix the bug with the...

Another attempt at 2178. This is a "minimal" patch designed to fix the bug with the minimum amount of new code. A better fix, suggested by Andre, would merge InsetTableCell and CellData into one class, in which case InsetTableCell::cell_data_ can be removed. A definite plus. I'll do that later if this works. But let's get it right first.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@23879 a592a061-630c-0410-9148-cb99ea01b6c8