Fix bug #6058: Change tracking not usable in version control.

Fix bug #6058: Change tracking not usable in version control.

This patch makes sure that there are minimal changes when loading and saving a file with change tracking.

- the authors are assigned a buffer_id, such that when the file is saved, they get the same id,
- the authorlist is sorted according to the buffer_id,
- the buffer_id is written to the file in the author list (file format change)
- the ids start with 1, because 0 is internally reserved for the current Author,
- when writing the file, the current author is assigned an id if he didn't already have it.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@30756 a592a061-630c-0410-9148-cb99ea01b6c8