Preliminaries for bug 900

Create WaveTrackCache as a utility class but don't use it anywhere yet.

The possible minor performance problem with effects is fixed by changes
in WaveTrack::GetBestBlockSize().

src/Spectrum.h

@@ -22,7 +22,7 @@
   calculates windowSize/2 frequency samples
 */
 
-bool ComputeSpectrum(float * data, int width, int windowSize,
+bool ComputeSpectrum(const float * data, int width, int windowSize,
                      double rate, float *out, bool autocorrelation,
                      int windowFunc = eWinFuncHanning);