Autosaves: disable autosave checks when multiple PhotoDemon instances are active

Relates to #141.

Without this change, each program instance will detect the other
instances' Undo/Redo data, assume an unclean shutdown occurred, and
wrongly prompt the user to initiate autosave recovery!
Undo and Autosave: integrate unique SessionID into all temp files

Fixes #141.

Multiple simultaneous PD instances should no longer cause trouble with
Undo/Redo or autosaving.

Similarly, failures in multiple simultaneous PD instances can now be
recovered by a single subsequent Autosave check (though I hope such a
thing never happens in real life!).