Consolidate `SequenceIdMappings` loading

[azriel91]

In GitLab by @azriel91 on Nov 29, 2019, 12:10

• AssetType::Object(..) and AssetType::Map store SequenceIdMappings in AssetSequenceIdMappings, as they only have one ItemId.

  This must be done prior to loading SpawnsSequenceHandles in AssetSequenceComponentLoadingSystem, as objects need to know the sequence ID of the objects they are spawning.

• AssetType::Ui loads SequenceIdMappings in AssetSequenceComponentLoadingSystem, as the SequenceIds need to be per BackgroundDefinition or UiDefinition for the indices in sequence component data (e.g. WaitSequenceHandles) to be correct.

This disparity works for now, but may be confusing. The difficulty in shifting the SequenceIdMappings loading for Objects is, Spawns can map from the AssetSlug in spawn config to AssetId, but not to an ItemId / SequenceId within that AssetId -- and ItemId does not exist until after AssetSequenceComponentLoadingSystem has run.