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_dialogs/test_dlg_projsettings.py

@@ -66,6 +66,8 @@ def testDlgProjSettings_Dialog(qtbot, monkeypatch, nwGUI, fncDir, fncProj, outDi
     # Get the dialog object
     monkeypatch.setattr(GuiProjectSettings, "exec_", lambda *a: None)
     monkeypatch.setattr(GuiProjectSettings, "result", lambda *a: QDialog.Accepted)
+    monkeypatch.setattr(nwGUI.docEditor.spEnchant, "listDictionaries", lambda: [("en", "none")])
+
     nwGUI.mainMenu.aProjectSettings.activate(QAction.Trigger)
     qtbot.waitUntil(lambda: getGuiItem("GuiProjectSettings") is not None, timeout=1000)