* Implemented drag-and-drop composition
* Added composition policy for tables
* Reimplemented elements pool in Vue
* No need to resolve all objects on the navigated path
* Only show elements pool in edit mode
* Remove old elements pool
* Updated legacy code to use composition policy API
* Keep object in sync when mutated
The composition policy now takes a child instance instead
of the child type, as in all cases we have access to the child
object.
This allows new-style objects to be contained by old-style objects.
Updated all composition policies to use standardized argument names
instead of `context` and `candidate`; this makes it easier to
understand.
Updated AddActionProvider to hardcode the object types supported.
