Duplication of assignment labels during serialization of edges with multiple updates

[gerking]

Serializing an edge with multiple comma-separated updates leads to an invalid duplication of the assignment label within the serialized XML (one separate label per assignment).

For example, parsing <label kind = "assignment" > x = 0, y = 1 </label > leads to

<label kind = "assignment" > x = 0 </label > 
<label kind = "assignment" > y = 1 </label >

in the serialized output. Most probably the same issue exists with selections.

This seems to be a known issue in Xtext for unordered groups, which are used by our grammar to enable an arbitrary order of labels for edges.