Add buttons to individually remove datasets, pron dicts and models

[mattchrlw]

Closes #138.

• Deletion is done in a cascading manner, that is:
  • Deleting a dataset deletes pron dicts and models that use it.
  • Deleting a pron dict deletes models that use it.
• Deletion is done at a file level, so clicking delete not only removes it from self.config but also uses shutil.rmtree.
• The UI for selection and deletion has also been standardised across the UI for consistency.
• Bug fixes where a user could proceed past the training stage where there is no datasets, for example.
• Loading logic has been adjusted so you can select a dataset, pron dict or model that has not been initialised.

[mattchrlw]

Pretty pictures!

[benfoley]

This is sooo good to have. Probably good to have a "are you sure" popup (as annoying as they are) but that can come later.

[benfoley]

lgtm, nice bonus bug fixes