Add a root key for widget types and branding registries

For now, com.nuxeo.studio.core.readers.BrandingReader and com.nuxeo.studio.core.readers.WidgetTypeDefReader class cannot be used on any JSON registries file because they just read all the first level JSON keys present in the file to transform them into BrandingContribution/WidgetTypeDefContribution.

The problem is that with the new Application Definition packages registries loading, there is only one studio-registries.json file per package and we try to use all the ResourceReader classes on it => it will create a Branding and WidgetType for events, facets, schema, etc...

So, waiting for this ticket to be done, these two reader classes will be ignored when loading studio-registries.json in a package