Bug 987642: improve (add) project PYTHONPATH update on src folder delete/rename (paste/move).

Implement a safer means of updating a project's PYTHONPATH when its source folders are
deleted or renamed, and also trigger a PYTHONPATH update when source folders are pasted
or moved into/from a project.

FEATURES THAT ARE MISSING FROM THIS PATCH:
-moving source folders with the Move wizard does not update PYTHONPATHs; only dragging does
-pasting/moving non-source folders that contain source folders doesn't update PYTHONPATHs