Remove the difflib spellchecker (#875)

* Delete simple spell checker
* Clean up the spell checker code
* Update tests
* Delete the bundled dictionaries
* Update documentation
* Use set() for user word list
* Fix tests so they pass also without dictionaries installed

tests/test_gui/test_gui_doceditor.py

@@ -229,7 +229,6 @@ def testGuiEditor_MetaData(qtbot, monkeypatch, nwGUI, nwMinimal):
     assert nwGUI.docEditor.docHandle() == sHandle
     assert nwGUI.docEditor.lastActive() > 0.0
     assert nwGUI.docEditor.isEmpty() is False
-    assert nwGUI.docEditor.currentDictionary() is not None
 
     # Cursor Position
     assert nwGUI.docEditor.setCursorPosition(None) is False