OPEN - task 1: Support for indexed value sets 
http://github.com/markus1978/emf-fragments/issues/issue/1

Extracted all functionality for extrinsic ID management into its own
class `ExtrinsicIdIndex`.
OPEN - task 1: Support for indexed value sets 
http://github.com/markus1978/emf-fragments/issues/issue/1

Refactored the `FragmentedModel` class.
OPEN - task 1: Support for indexed value sets 
http://github.com/markus1978/emf-fragments/issues/issue/1

Refactored the test meta-model, removed the old Container and Contents
test classes, add new features to `TestObject` for indexed value sets.
OPEN - task 1: Support for indexed value sets 
http://github.com/markus1978/emf-fragments/issues/issue/1

Added the necessary annotations.
OPEN - task 1: Support for indexed value sets 
http://github.com/markus1978/emf-fragments/issues/issue/1

Refactored the index classes. Common functionality is now distilled in
`AbstractValueSet` and its decendants.
Implemented the support for indexed value sets, but it is not tested
yet.
OPEN - task 1: Support for indexed value sets 
http://github.com/markus1978/emf-fragments/issues/issue/1

Implemented the grievances in this issue:
- refactored value set semantics (former value sets)
- moved from user site list implementation to internal setting delegate
based list
- added tests for value sets with containment and references
- implemented indexed list class, but not tested yet
- realized that tests must be improved
- many list operations are not supported