Migrate to UUIDs

[JanWittler]

This PR replaces hierarchical IDs (HIDs) with UUIDs for the internal consistency preservation. A UUID uniquely identifies an element but instead of a HID cannot be generated on the fly and is permanent. As such, any element must be registered to the UUIDResolver explicitly. Since elements are identified based on their identity and not their hierarchical position anymore, when copying a Resource, the UUIDResolver cannot be reused but a new one has to be created, matching the elements of the original resolver. As this case happens frequently, UUIDResolver::resolveResources is introduced.
This PR is introduced to enable the VirtualModel to identify all its elements and correspondences based on permanent identifiers, such that versioning, variants, and distributed functionalities are easier to support.

Since HIDs are still needed (in views), currently there are two similar class structures for HIDs and UUIDs. As next step, we should come up with one single interface to hide both functionalities. From my experience working on this PR, I suggest to aim for hiding almost all the currently exposed IDResolver logic and instead only provide functionality to assign IDs to a change or to resolve a change with the appropriate IDs assigned. As such, we should investigate whether changes can be made generic over the type of content (EObject, HID, UUID) they store (see #58).

Changes in detail

• introduced UUIDResolver and matching classes for assigning UUIDs to changes and applying and resolving changes containing UUIDs
• removed (UU)ID assignment from ChangeRecorder to reduce the occasions where an IDResolver has to be exposed
• Extend ChangeRecordingModelRepository with UUIDResolver
• Adapt ChangePublishingTestView to load the needed UUIDs into its own resolver when loading a resource

Follow-up tasks

• Provide single ID / UUID resolution interface #58
• Make EChange generic over its content type #59
• Unregister elements explicitly in UuidResolver #60

[JanWittler]

Missing tests for UUID resolver were added and code suggestions implemented.
I created issues for the follow-up tasks:

• Provide single ID / UUID resolution interface #58 (already started on branch id-uuid-interface)
• Make EChange generic over its content type #59 (already started on branch change-generic)
• Unregister elements explicitly in UuidResolver #60

As such, this PR (and its counterparts in the other repositories) is ready to merge. @vitruv-tools/maintainers

[h4uges]

Reviewed the changes of the implementation code (no tests)

[h4uges]

Reviewed the tests.

Suggestions are implemented, force pushing seems to cause some problems.