Update spellchecker APIs

* marks new properties in QQuickWebEngineProfile as FINAL
* removes QT_NO_SPELLCHECK from API headers
* renames spellCheckLanguages() to availableDictionaries()
* removes "togle spellcheck" methods and actions
* use WEBENGINE_CONFIG instead of CONFIG for disable the feature at
compile time: WEBENGINE_CONFIG+=no_spellcheck

Done-With: Peter Varga <pvarga@inf.u-szeged.hu>
Task-number: QTBUG-52371
Change-Id: I8c8eff497b9e7afe0cec2edc97dec248151487f2
Reviewed-by: Michael Brüning <michael.bruning@theqtcompany.com>