MPDConnection: Make MPDCommandResult a nested class and cleanup warnings.
MPDConnection: Make MPDCommandResult a nested class and cleanup warnings.

jcnoir/dmix,hurzl/dmix,abarisain/dmix,0359xiaodong/dmix,joansmith/dmix,hurzl/dmix,0359xiaodong/dmix,joansmith/dmix,abarisain/dmix,jcnoir/dmix