Automatically add the corresponding XML extension when declaring something in the registries

When declaring something in the registries, people do not understand necessarily that it also needs to be declared using an XML extension.

For instance, see:
https://answers.nuxeo.com/general/q/575ee95911ae48b79c04923aa1cb19a1/Problems-with-Facets-schema-prop

We could make it easier for people to use registries by generating the corresponding XML when needed.

TODO:

• Better inform users of the role of registries
  => Update help to explain the differences (text to be determined)

• Offer an option to generate the corresponding XML extension automatically
  => A checkbox should be added in the registries screen: "generate corresponding XML contribution"
  => It should be disabled by default, to prevent errors in existing projects

Behavior:

• If registry is empty or invalid, do not generate any XML
• If registry is filled and valid and checkbox is activated, generate the corresponding XML contribution in the extensions.xml file
• The generated contribution should be invisible to the user