Cherry-pick schema version 18 from features_key so beta testers can use it without getting out of sync with whatever happens to the schema in trunk.
Version bump to 1.12.0-alpha-pre.
Add Vedant Agarwala to the contributor list.
Bring back EnginePassthrough since we could use it for an auxiliary input.
Minor nits from the vinyl-passthrough merge. Avoid values() calls on QHash and duplicate contains/lookups on QHashes in SoundManager. Move AudioInput connections into PlayerManager, do it for every VINYLCONTROL AudioInput. If we don't get any beyond 2 it shouldn't matter, they just wont get connected/disconnected/received.
Remove Q_ASSERTs in EngineDeck. Support mono input (just in case).