Combine preferences callback and track-changes on defining query

[ralfhandl]

Current specification text suggests that preference callback can only be used

• on a GET request to a delta link

This suggests that two requests are necessary for being notified of changes:

• Send defining query with preference track-changes
• Send GET request to delta link with preference callback

Allow combining this into one request:

• Send defining query with both preference track-changes and preference callback

[HeikoTheissen]

It seems problematic that callback is a preference on its own which can appear together with another preference, either respond-async or track-changes. If callback was an attribute of these other preferences, it could appear in both in one request.