Add generic arguments to NotationElements

[CamilleLetavernier]

Currently, the only way to add custom information to the Notation model is to extend it with a new Ecore model, which may be tedious (and potentially overkill).

In GGraph, we already support GArgumentable, which is a generic map of properties. We should add similar support to NotationElement.

GArgumentable uses a StringToObjectMapEntry, which will probably not be supported by the current JsonPatchHelper, so we should also update the patch helper to support edition of this map from the Model server clients (Preferably in a generic way, i.e. without having to subclass the patch helper for this notation-specific use case).

• Let NotationElement extend GArgumentable from GGraph
• Update the JsonPatchHelper (https://github.com/eclipse-emfcloud/emfcloud-modelserver) to allow edition of this map of properties