Form Builder to support XBL components defining their required constraint

[avernet]

For this, we first need #4709 to be implemented. The XBL component would say:

<fb:metadata> 
    <fb:bind xxf:required-constraint="string() = 'true'"/>
</fb:metadata>

Form Builder would:

• Add an xxf:required-constraint="string() = 'true'" on the xf:bind when users mark an instance of that XBL component as required.
• Conversely, when finding an xxf:required-constraint="…" and the the XBL component has an <fb:bind xxf:required-constraint="…"/>, take the control to be required when the Control Settings dialog opens.

What happens if an XBL control changes its <fb:bind xxf:required-constraint="…"/>? When would the xxf:required-constraint="…" be updated? We already have this problem for the <fb:bind type="…"/>, so we should at the minimum have a consistent behavior in both cases.

+1 from user

[avernet]

+1 from user