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/common.py

@@ -31,8 +31,8 @@ import logging
 from datetime import datetime
 from configparser import ConfigParser
 
-from PyQt5.QtWidgets import qApp
 from PyQt5.QtCore import QCoreApplication
+from PyQt5.QtWidgets import qApp
 
 from novelwriter.enum import nwItemClass, nwItemType, nwItemLayout
 from novelwriter.error import logException