Disable inset-dissolve in table cells

Disable inset-dissolve in table cells

This is better than making sure that the inset has exactly one cell.

In the future, inset-dissolve could be extended to dissolve all the
cells contents in the enclosing text.

Note that this check does not appear in mathed, and using dissolve in
e.g. fraction inset may lead to data loss (only current cell contents
is preserved).

(cherry picked from commit 5ac3b880075794130492c2fe63c35104b207ad73)