This commit is partial effort toward more libsoxr integration, that implements Rob Sykes's latest patch. I have no idea whether the Linux-specific stuff from the patch (e.g., M4 file) is still correct in this.

For Resample.* and QualityPrefs.cpp, this commit has my restructuring for distinguishing constant-rate vs variable-rate resamplers more generally. I think it's complete and ready for const-rate, but I have more review and testing to do for the var-rate cases. 

Variable-rate resampling is not implemented here, so Time Tracks are still broken, but this is a milestone in getting to a more general and correct structure. 

Also I think this fixes AboutDialog issues Steve noticed.

src/AudioIO.h

@@ -38,7 +38,7 @@
 class AudioIO;
 class RingBuffer;
 class Mixer;
-class Resample;
+class ConstRateResample;
 class TimeTrack;
 class AudioThread;
 class Meter;
@@ -474,7 +474,7 @@ private:
 #ifdef EXPERIMENTAL_MIDI_OUT
    AudioThread         *mMidiThread;
 #endif
-   Resample          **mResample;
+   ConstRateResample **mResample;
    RingBuffer        **mCaptureBuffers;
    WaveTrackArray      mCaptureTracks;
    RingBuffer        **mPlaybackBuffers;