[DMN Designer] Do not group V&V messages by UUID

The V&V results are grouped by UUID. See attached screen-shot.

These items are reported individually by kie-dmn-validation but grouped by org.kie.workbench.common.dmn.showcase.backend.validation.ValidationServiceImpl in the standalone webapp however I fear Stunner's generic ValidationService does the same in Business Central and hence reporting DMN V&V per-line may be more troublesum.

See here that should more correctly be:-

    @Override
    public Collection<DiagramElementViolation<RuleViolation>> validate(Diagram diagram) {
        return domainViolations(diagram).stream()
                .filter(v -> Objects.nonNull(v.getUUID()))
                .filter(v -> !"null".equals(v.getUUID()))
                .map(v -> new ElementViolationImpl.Builder().setUuid(v.getUUID()).setDomainViolations(Collections.singletonList(v)).build())
                .collect(Collectors.toList());
    }

Acceptance criteria

• Create a DMN file
• Add a Decision node with no content
• Validate it
• Each validation message should be a separate item in the Alert panel