un-revert r30531, after Richard told me how to avoid the crash. Now, before

un-revert r30531, after Richard told me how to avoid the crash. Now, before
accepting changes in a clipboard copy (CutAndPaste.cpp), we set the buffer
of insets (and we reset them later).

Doing this makes sense because we know this is the only operation on these
out-of-document paragraphs that will require access to a buffer.

Also, this commit gets rid of one explicit test against ERT_CODE and
LISTING_CODE.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@30623 a592a061-630c-0410-9148-cb99ea01b6c8