... so that DeviceManager, DeviceToolbar, and PrefsDialog do not depend directly
on AudioIO.
But no function in the base class for starting streams, which would require
mention of Track types, which we want to avoid.
... New files, but (almost) empty; don't use the global variable gAudioIO,
but use one of two accessor function names (which are the same function for
now).
AudioIOBase will have fewer dependencies than AudioIO -- in particular, no
dependency on tracks.
It won't include StartStream. It will contain functions to query the
present state of streams, and device capabilities.
