Use IDs instead of names when writing to (MPS) properties/links

[nkoester]

modelix.core/model-api/src/commonMain/kotlin/org/modelix/model/api/INode.kt

Line 54 in ee71fe5

private fun IRole.key(): String = name // TODO use .getUID(), but this is a breaking change

This is a breaking change. Existing models cannot be read anymore.
This might be a good opportunity to introduce versioning and migration of models.