Fix for bugs 2199 and 3667. Most of the changes were to the logic. Buffers

Fix for bugs 2199 and 3667. Most of the changes were to the logic. Buffers
that were opened for possible included LyX files need to be closed if those
files cannot be loaded, for example. A bit more work was needed to allow a
LyX file to include itself as a verbatim or listings without trying to load
itself and without relying upon loadIfNeeded() to report the problem.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@18494 a592a061-630c-0410-9148-cb99ea01b6c8