Spellcheck: port to EditorSpellChecker
As part of the GTK 4 port, we'll lose GSpell integration.
We can move some of it to use EditorSpellChecker as part of gnome-text-editor, but it will need a "navigator" interface added if we are to go in the same route as were before.
But we can totally drop that and just do inline spellchecking and very few people notice likely.