New attempt to fix Pavel's crash, this modifies r40206.

New attempt to fix Pavel's crash, this modifies r40206.

The global list of cloned buffers was a bad idea. We don't have multiple
export threads, but we can have multiple autosave threads. So now we keep
more than one of these lists at the same time, and each cloned buffer has
a pointer to the relevant list. Note that this would make multiple export
possible, if we wanted to do that at some time.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/branches/BRANCH_2_0_X@40229 a592a061-630c-0410-9148-cb99ea01b6c8