Multi-scene support for storage systems

[SugoiDev]

This is a initial working version of a multi-scene storage system. FI's backup and storage systems are broken with multi-scene. This PR brings initial support for multi-scene setups.

I would appreciate some code review and ideas for dealing with Unity's cross-scene weirdness, if anyone has know-how in that area.

Warning

• This pull-request is not considered ready, and should not be merged to master. The underlying systems seem to be working under my limited testing, but it could have edge cases that aren't account for. There's also rough spots (see below) with Unity's cross-scene references.

• Those changes are probably not compatible with older Unity versions. I only tested on Unity 5.5.2.

Overview of changes

• abstracted the ideas for storages that were used only for the backups system into a new abstract type fiBaseStorageComponent<T>. This object has methods to refresh the storage references, as well as create new ones if they become stale or corrupted. It does not perform cleanup based on name.

• fiStorageManager in the BackupService has a member of that type, as well as fiPersistentEditorStorage.

• for fiPersistentEditorStorage, create a fiPersistentEditorStorageComponent that is just a MonoBehavior shell so we can use FindObjectsOfType.

Most other changes are just related to API change (like using instance IDs in certain places, since the actual references would be dead when exiting playmode or changing serialization type). Also adding a null-guard for the only usage of Read method, that can now return a default(T).

What isn't good yet

Note: Unity will show warnings when we move a FI object from a scene to another. This is because we're storing a reference to the object in the scene storage(s), and Unity forbids cross-scene references (will show a "scene mismatch" message on the object's field, as well as warnings in the console).

• I managed to get the references from the fiBackupSceneStorage properly (Unity will still show warnings), but I found no way to migrate the references stored in the fiPersistentEditorStorage.

• This is a potential problem-case that I didn't test: In playmode, the user can load scenes in-game, then backup a object in the newly loaded scene. When exiting playmode, we would not be able to migrate that object's backups. I imagine that, with the current way the fiStorageManager.MigrateStorage() is called, the migration would occur the next time the scene is loaded. Again, I did not test this.

[jacobdufault]

How has this been working out for you? If you have a more recent version feel free to repush and ping and I'll take a deeper look at the changes.

[SugoiDev]

I've been using it ever since without issues in a multi-scene heavy setup. I don't do a love of cross-scene objects moving, but that was the only thing with some issues still (unity warnings for cross-scene refs).

This cross-scene reference madness is probably because Unity lacks actual persistent object IDs in its pseudo-ECS.

[SugoiDev]

I might have some extra changes here, so give me a few days before you review/merge.

[SugoiDev]

Alright. I've added a few missing files from my current project and fixed the unicode characters in the backup context menus.