Flowable-Process-Validation: No validation error for unattached timer boundary event

When moving a timer boundary event in the modeler it is very easy to unattach it by mistake.

The model is still considered valid, but fails on deployment with an error in the log:

```
ERROR org.flowable.common.rest.exception.BaseExceptionHandlerAdvice - Unhandled exception
org.flowable.engine.common.api.FlowableException: cvc-complex-type.4: Attribute 'attachedToRef' must appear on element 'boundaryEvent'.
```

It would be helpful if flowable process validation would detect these kind of errors.

(related issue: https://github.com/flowable/flowable-engine/issues/544)