DD — Simplify DI metamodel

The DI metamodel can be simplified by removing
planes (diagrams can be nested) and leaving
packaging out of scope (DiagramCollection, and
the association between Diagram and Style).

The DI metamodel is simplified by merging planes and plane elements into diagrams and diagram elements, respectively, and removing labels and diagram collections
Planes are removed. These were to support nested diagrams, but it was decided the additional layer of terminology wasn't necessary, especially when some communities simply call them "nested diagrams". In the revised text, diagrams can be nested. Top level diagrams are those diagrams that happen not to be nested. We discussed various possibilities for modeling this, and settled on treating everything as a nestable diagram element, including diagrams, which are taken as a kind of shape. This means diagrams can have model elements, for example, as in UML composite structure. Top level diagrams won't use bounds. Diagrams can be connected by edges, for diagrams of diagrams.
Labels are removed. The only difference from shapes is they are rendered as strings, which would often be determined by the mapping to DG. Language-specific DI's can define these as kinds of shapes if needed.
It was decided that packaging should be out of scope, because domain languages usually have their own packaging mechanisms, resulting in these changes:
The existing association between Diagram and Style is removed, because it was only packaging styles, it didn't mean styles applied to the diagram, like the association between DiagramElement and Style, which Diagram now inherits.
DiagramCollection is removed, and its packaging associations to Style.
Packaging of styles is left in Diagram Graphics, because DG is not intended to extend a domain language that might have its own packaging mechanism.
Updates to the Annexes for these changes will be addressed in separate issues.