Fix bug 5146. The problem arose when attempting to paste normal text into

Fix bug 5146. The problem arose when attempting to paste normal text into
a multicell selection. But the more I thought about this, the more it seemed
that pasting into multicell selections shouldn't be allowed. What exactly
ought to happen if the user selects a 2x3 grid of cells and then tries to
paste a 5x5 group of cells? LyX, anyway, just ignores the selection in this
case, which is not what the user will expect. So let's just not go there.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@26099 a592a061-630c-0410-9148-cb99ea01b6c8