API V2: The patch operation "remove" doesn't remove cross references

[CamilleLetavernier]

When using the Patch operation "remove" to delete an object from the model, cross references are not unset. This is a bug in the JsonPatchHelper, that returns a "Remove" command instead of a "Delete" command.

[CamilleLetavernier]

Note: this is only an issue for the "EMF-like" Json Patches, using this syntax to delete an object from the model:

[{
   "op": "remove",
   "path": "mymodel.xmi#objectId"
}]

Which corresponds to an EMF "Delete Command".

For standard Json Patches, the semantic is instead closer to the EMF "Remove Command":

[{
   "op": "remove",
   "path": "parentid/reference/childIndex"
}]

or an unset Command:

[{
   "op": "remove",
   "path": "parentid/reference"
}]

In these cases, we don't want to remove cross-references.

However, it is unclear to me what the expected behavior is, when unsetting/removing an object from a Containment reference (when reference is a Containment ref). The client might want to provide a patch with multiple operations to unset the cross-references, or to ignore the EMF semantics and leave the responsibility to the EMF server. We should define the expected behavior in this case.

IMO, it makes sense to treat removal from a Containment reference like a Deletion, and also clean-up all cross references. If the Client intends to reuse the Object, it would use a "Move" operation instead (Although this isn't supported yet: issue #179), or an "Add" to the new location (EMF is then responsible for removing the object from its former parent, and adding it to the new parent).