Improve performance of DiagramMigrationHook

The DiagramMigrationHook is inefficient for creating transactional
editing domains, which is very costly as these domains install adapters
to each object. The DiagramMigrationHook now creates a new transactional
editing domain for each diagram and disposes it again.

Instead the editing domain should only be created if needed and be
reused across migrating all diagrams.

Change-Id: I3abda32c4248e853c6568cd47f2ba3b3a0e6a161
Signed-off-by: Philip Langer <planger@eclipsesource.com>