[OAuth] Qt##7 removal markings

There are several candidates for Qt7 removal or refactoring, which cannot be done in a BC/SC manner.

This task is putting these markers in place. The reason for having a separate item is to accrue the list
while working on other improvements; a prominent source for such markings.

The accruing list:

• Remove the QAbstractOAuth get() post() etc methods and document how to use access tokens in network requests
• Remove the undocumented QOAuthOobReplyHandler class and reorganise the few functionalities it has somewhere else
• Drop OAuth1 support
• Simplify the inheritance hierarchy which should be possible when dropping the OAuth1 support, and dropping "Oob"
• addhere