Fix alignment rendering of multirow in LyX (#8976)

Fix alignment rendering of multirow in LyX (#8976)

Multirow cells now have the same alignment rendering in LyX
as in the output. The alignment of a multirow can change as
long as the column is not of fixed width. If the column is
of fixed width, the multirow is left-aligned.

When setting a multirow, the alignment is copied from
the last cell in the selection.