Abstract virtual AudioIOBase::StopStream...

... 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.

src/AudioIO.h

@@ -586,7 +586,7 @@ public:
     * Does quite a bit of housekeeping, including switching off monitoring,
     * flushing recording buffers out to wave tracks, and applies latency
     * correction to recorded tracks if necessary */
-   void StopStream();
+   void StopStream() override;
    /** \brief Move the playback / recording position of the current stream
     * by the specified amount from where it is now */
    void SeekStream(double seconds) { mSeek = seconds; }