Add UIKit state restoration

[farktronix]

Add UIKit state restoration to Relisten so that the currently playing track and viewed show is preserved.

[farktronix]

This is half implemented on the StateRestoration branch. I'm going to keep a list here of tasks that still need to be done:

• [x] Add restoration data to all of the Relisten View Controllers
• [ ] Add restoration data to AGAudioPlayer
• [ ] Add modelIdentifierForElement/indexPathForElement to HorizontalShowCollectionCellNode
• [ ] Hook up indexPathForElement in ArtistsViewController to return an actual artist row
• [x] Merge the necessary changes into Texture (https://github.com/TextureGroup/Texture/pull/1354)
• [ ] Move state restoration code into their own extension files

[farktronix]

There's also a new TestFlight beta for the StateRestoration branch. Anyone that's interested can add themselves to it here