Make use of label edit validation

[planger]

GLSP supports validating the label during label editing. The validation can be performed on the server. See for instance https://github.com/eclipsesource/graphical-lsp/blob/master/server/example/workflow-example/src/main/java/com/eclipsesource/glsp/example/workflow/labeledit/WorkflowLabelEditValidator.java

I think it would be helpful to use this feature for editing attributes (to check if the entered name and type are valid).