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/reference/baseConfig_novelwriter.conf

@@ -1,5 +1,5 @@
 [Main]
-timestamp = 2021-08-26 17:48:02
+timestamp = 2021-08-30 21:27:27
 theme = default
 syntax = default_light
 icons = typicons_light
@@ -53,7 +53,6 @@ fmtdoublequote = “, ”
 fmtpadbefore = 
 fmtpadafter = 
 fmtpadthin = False
-spelltool = internal
 spellcheck = en
 showtabsnspaces = False
 showlineendings = False