Add bidirectional references

[CamilleLetavernier]

Ecore supports bidrectional references: two distinct references with an "opposite" attribute referencing each other (e.g. Parent/Child references are bidirectional)

Currently, such bidirectional references are displayed with two distinct edges. We should use a single edge, and place the label for each reference on the side of the edge, rather than having overlapping labels on the center:

Current state: 2 overlapping Edges and overlapping labels:

Expected state: 1 single Edge with labels placed next to the referenced elements: