If a feature dependency of ItemMultiMaterial was disabled, the returned value was never assigned with the constructed. Pulling out the construction and setting it before checking it, prevents the NPE and also matches the behaviour in ItemMultiPart, where parts are constructed, but never registered.
Deprecates the old usage of the AEItemDefinitions via the direct method access of
* blocks()
* parts()
* items()
* materials()
and thus use the new re-direct via definitions().
All definitions are now initialized, no matter what. But SubItems, Items and Blocks are not registered, if by chance are disabled.
To integrate these into the current system, some changes to the background had to be done, especially to the feature handler. It now uses an interface to work against which you can implement to get your own feature handler instead modifying the base one and add several special cases code
