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.
Bug 987642: perform PYTHONPATH updates for drag & drop with folders that contain src folders.

Previously, using a drag & drop operation on a non-source folder that contained source folders
did not trigger PYTHONPATH updates on the source/destination projects. Resolve this issue.
Use /${PROJECT_DIR_NAME} when creating/editing source folders, and don't auto-resolve PYTHONPATHS.

1. More scenarios that create or edit source folders (such as adding a new/linked src folder,
pasting & moving a src folder) use /${PROJECT_DIR_NAME} when updating their projects' PYTHONPATH.
2. Deleting/renaming/pasting/moving source folders no longer resolves the entire PYTHONPATH of
host/destination projects.