Preliminary function argument list changes and comments, for scrubbing project

In particular, use an options structure for AudioIO::StartStream to simplify
calls

ControlToolBar::PlayPlayRegion also takes that structure as an argument, and a
SelectedRegion instead of two times

And other changes

src/Mix.h

@@ -67,12 +67,29 @@ class AUDACITY_DLL_API MixerSpec
 
 class AUDACITY_DLL_API Mixer {
  public:
-   //
+
+    // An argument to Mixer's constructor
+    class WarpOptions
+    {
+    public:
+       explicit WarpOptions(TimeTrack *t)
+          : timeTrack(t), minSpeed(0.0), maxSpeed(0.0)
+       {}
+
+       WarpOptions(double min, double max);
+
+    private:
+       friend class Mixer;
+       TimeTrack *timeTrack;
+       double minSpeed, maxSpeed;
+    };
+
+    //
    // Constructor / Destructor
    //
 
    Mixer(int numInputTracks, WaveTrack **inputTracks,
-         TimeTrack *timeTrack,
+         const WarpOptions &warpOptions,
          double startTime, double stopTime,
          int numOutChannels, int outBufferSize, bool outInterleaved,
          double outRate, sampleFormat outFormat,
@@ -129,6 +146,7 @@ class AUDACITY_DLL_API Mixer {
    // Input
    int              mNumInputTracks;
    WaveTrack      **mInputTrack;
+   bool             mbVariableRates;
    TimeTrack       *mTimeTrack;
    sampleCount     *mSamplePos;
    bool             mApplyTrackGains;