Created a new AssetLocation.assertIdentifierCorrectlyNamespaced() method
so that content plug-ins don't all need to replicate this same logic
(often incorrectly) themselves, but also so that it can now also make
use a new AssetContainer.isNamespaceEnforced() method that allows asset
containers to specify whether namespace enforcement is enabled or not.
Converted content plug-ins to start making use of new
AssetLocation.assertIdentifierCorrectlyNamespaced() method.