Getting rid of annotations on the native side

[felix-ht]

With the addition of geojson layers, we now have have the option to implement the annotation manager code in dart. This would significantly reduce the required plugin surface area.

Pro

• The dart code for this pretty much exists already in the Web bindings for annotations
• easier to move forward (say to version 10 of the mapbox sdk - Migration to 10 #691) as we "only" have to maintain the code related to layers and not the annotation manager
• More consistent behavior between targets - less non dart code
• This would allows users to add the new (to be created) AnnotationManger, in whatever order or below whatever layer they desire. No need anymore for clunky stuff like the annotation order.
• Keeping the current API stable is possible as well

Con

• Drag and drop might prove to be tricky (mostly because of responsiveness)

What additional stuff would we need on the native side:

• option to update a single feature in a geojson layer