Refactor out clsid in CropObjects

[hajicj]

It does not ever get used for anything useful, instead introducing an extra dependency on the MLClassList.

However, there are more places where this change needs to be made within MUSCIMarker.

[hajicj]

Places to change:

(Note: we don't need to phase clsid out of MLClasses, it's a useful tool for sorting them. However, we want to phase it out of CropObjects.)

• [x] CropObject initialization
• [x] CropObject export
• [x] App: generate_cropobject_from...() functions
• [x] apply_mlclass_to_selection()
• [x] selectable_cropobject_converter() uses clsid as index into mlclass color dict
• [x] CropObjectView tracking on toggle_class_selection, do_class_selection and a ton of others (can be easily changed to clsname)
• [x] CropObjectView: MLClass selection spinner (can be changed to use model.mlclasses_by_name[clsname])
• [x] CropObjectView: do_mlclass_selection() equality check (can be changed to clsname)
• [x] CropObjectView: set_mlclass() - can be deleted
• [x] CropObjectView: split(), which leads to App.add_cropobject_from_selection()
• [x] Model tracking
• [x] Model: CropObject validation (can be rewritten to names)
• [x] muscimarker_io.position_cropobject_list_by_muscimage()
• [x] muscimarker_io.render_annotations() (rewrite mlclass_dict to be indexed by clsname)
• [ ]