Ensure that IDs have been assigned to all new model elements before they are hooked to the project / resource
- Rationale
  - Model elements for which no ID has been assigned, have id 0 (which is the default value). 
  - When the new element is hooked to the resource, ResourceUtils.KernelResourceFactory.KernelXMIResource.getId() is called, which internally calls setId() to synchronize the XMI ID with the AF3 ID. In case the ID is still 0, this implicit call will corrupt the ID -> model object map (by override the mapping for key 0).
- Changes
  - ElementCompositorService: Assign IDs *before* hooking new element to the container (but of course using the container to determine the next free ID)
  - ModelElementCompositorBase: Assign IDs to newly instantiated annotations