Convert EnableCache, DisableCache, and SentinelActions to use input encoder/decoders. Clean up some header dependencies uncovered by adding cpp file for SentinelActions.
Convert Initialize{Window,Focus}, FocusSet{Focus,Active} to InputCoder. Remove dead code and unnecessary forward declarations. Add InputDecoder::getBoolean().
Convert PlaybackError and InterpretedKeyCommands (sans-decode) to InputCoder.