BUG: Internal Paths are "orphaned" when it's pattern piece is deleted

[slspencer]

• Describe the bug When a pattern piece is deleted, the internal paths aren't deleted.

• How To Reproduce the bug:

  In Draft mode: - Create a pattern piece.
  - Add a dart then "true" the darts with the True Darts tool.
  - Create a pattern piece detail.
  - Add an internal path for the dart to the detail. Go to Detail mode:
  - Delete the detail.
  Return to Draft mode:
  - Try to delete any item in the Dart. It should be able to be deleted but it's referenced in an unused "orphaned" Internal Path object. - Orphaned Internal Paths can only be deleted by opening the XML file in an editor and deleting it manually.

• What should have happened When deleting a pattern piece, check it's Internal Paths for number of occurrences in the file and delete the Internal Paths that occur only once.

• Screenshots

• What's your Desktop OS? Windows 11

• What's your Seamly version from Help/About? Seamly 2023.2.12.2224