XClass protection should be consistent with extension pages protection

We already had discussion and vote about the possible protection levels and the best default when extension pages protection was implemented so we should follow it for XClasses too IMO.

I think the best would be to duplicate extension.xar.protection property with the same default into a more generic page protection property and make extension.xar.protection inherit it by default. That way page protection is consistent by default and people can set a special protection for extension page if needed (we already have the property so we have to maintain it anyway).