Address bug 987642: deleting/renaming a project's source folder updates its PYTHONPATH.

Deleting or renaming a project's source folder no longer reports that the folder is missing.
Performing either operation updates the project's PYTHONPATH accordingly.
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
Prevent errors when manipulating source folders from non-Python projects.

Prevent errors with project PYTHONPATH updating when deleting/renaming/pasting/moving
source folders from non-Python projects.

Deleting/renaming/pasting/moving a project's source folders now always correctly updates
the project's PYTHONPATH.
Bug 987642: complete PYTHONPATH updating features for moving src folders with Move wizard.

Moving source folders with right click->Move... now correctly updates the PYTHONPATHs of all
projects involved in the move.