This change aims to allow TES4/TE5 content to OpenMW. i.e. a standalone TES4 would be implemented quite differently. That said, the key changes are:
* Use pointers rather than references for ESM readers so that they can be switched to another variant on the fly.
* Content file dependencies to be checked within each group (only 3 groups for now, TES3/TES4/TES5)