[450949] Avoid shortcuts in DefaultDiffEngine for certain FeatureMaps

Changes in FeatureMaps should be merged via FeatureMapChanges in the
FeatureMapChangeMerger. In cases of added / removed elements the
DefaultDiffEngine prevents the calculation of required FeatureMapChanges
since they only affect attributes of those elements. This leads to
ordering problems since the ReferenceChangeMerger can not handle
FeatureMaps properly. This fix removes the calculation prevention for
these cases.

Includes testcases for this issue.

Bug: 450949
Signed-off-by: Stefan Dirix <sdirix@eclipsesource.com>
Change-Id: Ie21a54c2e08b27bcc421f1c28d42e542407b990d
The ConflictMerger ranking should be above that of most others

As it was, the conflictmerger was nerver called for custom diff types
such as UML reference changes, which were merge through
UMLReferenceChangeMerger. This lead merge operations through the UI to
merge one side of the conflict while ignoring the other (instead of
rejecting it). The UMLReferenceChangeMerge (or other custom mergers)
will still be called, but only through the proxy of the ConflictMerger.

Change-Id: I0625b3c3b96f967c8b86edee360cc6d1b355c28c