make AnnotationContext a multi-object of ClassDescriptions instead of a field

[Wumpf]

Pro:

• makes for a less confusing api: Right now one specifies the path an annotation context is added to whereas all (?) other things take a path that specifies where the logged object lives
• allows to name your annotation context (right now it’s hardcoded to _annotation_context!)
• makes it more straight forward fix ui issues from the keypoint api - we can then show keypoints only when a certain ClassDescription is displayed
• “annotation context” goes away, it’s just “a bunch of ClassDescriptions”
• paves the way for having multiple sets of class descriptors (I might have several kinds of class ids at once because I ran two classifies)

Con:

• need to overhaul the way class descriptions are queried again, unclear to me how hard/easy
• how hard is it to keep the table we have on annotation-context right now? need to query children when annotation context object is selected
• annotations not being a property of a path might make it confusing when they apply
• could do something else with precious time

Internal slack thread https://rerunio.slack.com/archives/C033K5VS2KD/p1669625884256649

[Wumpf]

work for this started on andreas/classdescriptors-as-object

[Wumpf]

this is very outdated by now. Lots has moved around there and

• Improving on Annotation Context (meta) #2405
  captures things better anyways