Checkboxes: Don't set required attribute

[lorenzulrich]

If the Checkboxes Fusion definition, the attribute required is set for every checkbox if the "required" property is set. This means that checking all boxes will be required. This may work for a single checkbox (e.g. "accept terms and conditions"), but not for more than one.

I suggest to either remove this or only set it if there is only one option.

[mficzel]

It makes sense to:

1. Hide the required option in inspector if more than 1 option is defined
2. Only render the required attribute if required is set and only a single option is given