Fix and optimise code (#904)

* Fix bug in early error reporting in main init
* Update docstrings and optimise code in GuiMain
* Update docstrings and optimise code in Config
* Update docstrings and optimise code in common module
* Update docstrings and optimise code in main project classes
* Update the OptionState class
* Update the spell checker class
* Update the file converter classes and extend tests
* Update the about, merge, split and item editor classes and extend tests
* Update item editor test
* Update about dialog tests
* Some minor test cleanup
* Fix typo and add clarification in contributing guide

novelwriter/dialogs/docmerge.py

@@ -55,7 +55,7 @@ class GuiDocMerge(QDialog):
         self.outerBox = QVBoxLayout()
         self.setWindowTitle(self.tr("Merge Documents"))
 
-        self.headLabel = QLabel("<b>%s</b>" % self.tr("Documents to Merge"))
+        self.headLabel = QLabel("<b>{0}</b>".format(self.tr("Documents to Merge")))
         self.helpLabel = QHelpLabel(
             self.tr("Drag and drop items to change the order."), self.theParent.theTheme.helpText
         )