API: Added missing factory methods to AssetService.
API: Type hinting for assets.
API: Asset does not know about Variant => VariantService->getVariant().
SPI: $alwaysAvailable to determine if a variant is always generated.
SPI: Moved filter definition to the end.
SPI: Split up asset handler methods.
SPI: Added $typeHint to handler generate method.
SPI: Added missing methods to storage handler.
SPI: Asset variants need $storageUri and $webUri.
SPI: Storage identifier for VariantDefinition.