Refactor core and base classes (#931)

* Improve the NWIndex class
* Improve the NWItem and NWDoc classes
* Make some optimisations in projects and update tests
* Minor changes to string formatting in main source file
* Add some more protection to core classes
* Make all converter class attributes private

novelwriter/core/options.py

@@ -123,7 +123,7 @@ class OptionState():
     ##
 
     def setValue(self, group, name, value):
-        """Saves a value, with a given group and name.
+        """Save a value, with a given group and name.
         """
         if group not in VALID_MAP:
             logger.error("Unknown option group '%s'", group)