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

novelwriter/core/__init__.py

@@ -22,7 +22,7 @@ along with this program. If not, see <https://www.gnu.org/licenses/>.
 from novelwriter.core.document import NWDoc
 from novelwriter.core.index import NWIndex, countWords
 from novelwriter.core.project import NWProject
-from novelwriter.core.spellcheck import NWSpellCheck, NWSpellEnchant, NWSpellSimple
+from novelwriter.core.spellcheck import NWSpellEnchant
 from novelwriter.core.tohtml import ToHtml
 from novelwriter.core.toodt import ToOdt
 from novelwriter.core.tomd import ToMarkdown
@@ -32,9 +32,7 @@ __all__ = [
     "NWDoc",
     "NWIndex",
     "NWProject",
-    "NWSpellCheck",
     "NWSpellEnchant",
-    "NWSpellSimple",
     "ToHtml",
     "ToOdt",
     "ToMarkdown",