Initial issue title: Field Validation: Don't ignore 'required' property on the CompositeField itself.
Setting on the CompositeFieldDefinition the required property it is ignored.
So if one needs to ensure that all fields in the composite need to be filled out, one must set to all sub fields a required property.

Much easier would be if the required could be set on the composite itself.

Use case definition:
If the Composite field sets required=true, every sub field in the composite needs a value.

Problem:
CompositeFieldDefinition does not implement isValid(), and the super's isValid is only checking the validation of the sub fields.

This code solves the problem, I included a patch.

@Override
    public boolean isValid() {
        boolean isValid = super.isValid();
        
        if (this.isRequired()) {
            if (isEmpty()) {
               isValid = false;
            }
        }
        return isValid;
    }

UPD:

• This looks like a part of more fundamental problem: the validate() method of com.vaadin.ui.AbstractField is not invoked at all.
  • If it was invoked then the standard isRequired()/isEmpty() check would be handled.
  • It is supposed to be called within com.vaadin.ui.AbstractField#isValid but AbstractCustomMultiField impl doesn't call super method.
  • As another side effect the value validators of the AbstractCustomMultiField wouldn't be triggered since that also happens in the validate() method.

Proposed solution:

• Simply call super.isValid() in AbstractCustomMultiField#isValid instead of assigning true as a default value.