Use custom commands to enable complete editing

[ndoschek]

At the moment, it is not possible to edit names or add/delete nodes via other editing capabilities (property-view and theia-tree-editor) than the GLSP diagram.
This is due to the attached notation resource handling of which the two editors are not aware of.
It could be useful to realize this via custom commands, which the model server can interpret and cover the necessary notation updates if they are not provided by the triggering client.