- By default, EMF uses fragment-path based links to persist EReferences. While this works fine for standalone model resources, it can cause inconsistencies in case there are multiple resource models that contain cross-resource references. (I.e., if the structure of the AF3 model resource changes, the links from the external model resource can become invalid)
- The recommended way to work around this is to configure EMF to use IDs to encode EReferences.

Solution:
- Change persistence of AF3 model resources to encode references using an (extrinsic) xmi:id, that is identical to the model element's org.fortiss.tooling.kernel.model.IIdLabled.id. A dedicated XMIResource type for AF3 model resources is used (which is created by a resource factory registered for *.af3_23 files)
- The reason for not using IIdLabled.id as intrinsic ID is that the implemented solution using xmi:id is both forward and backward compatible
- At the price of losing forward compatibility, the IIdLabled.id could be changed to a derived, volatile EAttribute that simply returns the value of xmi:id
- Add EContentAdapter to sync xmi:id and IIdLabled.id
- Ensure uniqueness of IDs (that is now much more crucial) during when loading and saving as well as importing and exporting models (and print a warning to the console in case duplicates have been detected and fixed)
- As an additional safety layer, add constraint checkers to ensure uniqueness of IDs and consistency of IIdLabled.id and xmi:id

Related changes:
- Speed up loading and saving of XMI resources by setting corresponding XMLResource options
- Ensure that IDs are assigned to annotations as soon as they are instantiated
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- GSNPositionedEditPartBase*: Avoid code duplication by pushing refreshDecorationFigure() to PositionedEditPartBase
- PositionedEditPartBase.refreshDecorationFigure()
  - Avoid SWT Image leak by reusing the same icons
  - Add more null-checks to implementation
- MarkerViewPart -> column updater: Avoid SWT Image leak by disposing old image before setting a new one

- Undo last two changes: will be handled by ElementCompositorService (in the kernel)

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- Assign unique IDs to annotations as soon as the are instantiated (before, annotations relied on the ID-fixup that is triggered when the model is persisted)
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- Keep order of elements to be selected (e.g., EENum values) etc. by using a LinkedHashMap instead of a TreeMap

Fixes a bug in the EStructuralFeatureValueProvider that caused the Enum filter list to be applied on objects that should not be filtered.

Adds the possibility to filter the list of available choices for an enum-type annotation via the annotation's value provider.
Therefore, a map containing the class of the to be filtered model elements and the allowed enum literal must be given in the constructor of the value provider.

DerivedAnnotationValueProviderBase.setAnnotationValue(): Convert String-encoded input to actual data type

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generalize RequirementSpecificationUtils.removeRequirementSpecifications() to 
BaseModelElementUtils.removeModelElementReferenceSpecifications()
  
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