UUID Resolver Element Cleanup and Placement in Separate ResourceSet

[HeikoKlare]

This PR adds functionality for cleaning up and saving the UuidGeneratorAndResolverImpl while separating it from models resources and effects to them. In detail it provides the following:

• Functionality for cleaning up removed elements in UuidGeneratorAndResolverImpl, i.e., removing elements that are not placed in a resource anymore, which is automatically executed on saving the instance.
• Placement of the Uuid resource in a separate resource set, which was placed in the models' resource set before. This is useful because some Ecore functionality (such as EcoreUtil.delete) uses the cross referencer for a complete resource set, thus unintentionally affecting the UUID model. For example, calling EcoreUtil.delete on a model object will also remove it from the repository of the UUID resolver if placed in the same resource set. See also #441, for which this is a replacement. In addition, the resource in now managed within the UuidGeneratorAndResolverImpl (as it uses a separate resource set anyway).
• Have the resource of the UuidGeneratorAndResolver automatically saved (and cleaned up on save) in the ResourceRepositoryImpl. This only possible because of element cleanup, as otherwise orphan elements without a resource prevent the serialization of the resource.
• Proper implementation of load functionality for the UuidGeneratorAndResolverImpl, resolving the elements in the models' resource set. This also serves as a loading functionality for the model resources of a VirtualModel, such that storing them separately is not necessary. The UUID storage contains all necessary information about model resources to be loaded.
• Addition of UUID test cases for element movement, element deletion, resolver cleanup and saving, as well as different loading scenarios.

Replaces and closes #441. Fixes #298.

[HeikoKlare]

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