API V2: Specify a label for Json Patches

[CamilleLetavernier]

This is a follow-up for #150 / #151

Json Patches only specify the changes to be applied on the model. However, in an Editor, we also like to have label for edit-operations, which can be displayed e.g. in the "Undo/Redo" menu.

At the moment, all Commands derived from Json Patches use a predefined label ("Json Patch"), which is unhelpful.

We should consider a way to specify labels for edit operations. It can be either shared for both CCommand and Json Patch, or specific to Json Patch.

We could add it to the endpoint, e.g.:

PATCH .../models?modeluri=uri&label=My%20Custom%Label
BODY ...

Or make it specific to Json Patch, by adding more data next to the patch itself:

PATCH .../models?modeluri=uri
BODY data: { 
    patch: ["..."],
    label: "My Custom Label"
}

CCommands may already specify a label, so both options make sense. The first one is more consistent, and the second option avoids redundant labels for CCommands, which could be confusing (as only one label would actually be taken into account).