(updated) PYTHONPATH update on source folder delete/rename/paste/move

This includes all of the fixes I made so far to PYTHONPATH updates when modifying project source folders. The only feature left for me to implement is handling updates when dragging-and-dropping a non-source folder that has source folders inside it (since _those_ source folders get ignored).

PS: if you would rather have me squash these in a single commit later, please let me know. I'm keeping them separate for now to keep things clear.