MigrationService: Reimplement the internal logic

* The service now checks a given input element for the presence of
  migrateable elements if migration providers are queried. By that,
  the registration w.r.t. an element type actually makes sense. Before,
  only the needsMigration(...) method was evaluated.
* Simplify the needsMigration(...) method to indicate whether any
  migration provider applicable for the given input reports the need
  for migration.
* Simplify the migrate(...) logic that will now also report failed
  migrations: the need for migration is checked after the actual
  migration.
* Do not register migrators for ITopLevelElements any more! Instead,
  declare the type that is really relevant for the migration. If you
  need to be launched in every case (avoid that!!!!) one can register
  for FileProject or EObject.

Issue-Ref: 3806
Issue-Url: https://af3-developer.fortiss.org/issues/3806


Signed-off-by: Alexander Diewald <diewald@fortiss.org>