Add some more wishlist items to TODO.
Make an extractor head know about its parent extractor.
Make a parent extractor mandatory.
Fix failing test due to recent extractor model change.
Create an add extractor head button.
Allow user to add an extractor head.
Allow user to delete an extractor head using delete tool.
Allow user to move an extractor head using move tool.
Extractor heads are drawn beneath links and buildings.
Extractor heads only overlap with other extractor heads.