Remove many mentions of sampleCount with auto and decltype...

... This makes much code agnostic about how other things (functions and
arguments) are typed.

Many of these neeed to become size_t instead of sampleCount.

src/effects/ChangePitch.cpp

@@ -417,7 +417,7 @@ void EffectChangePitch::DeduceFrequencies()
 
       double trackStart = track->GetStartTime();
       double t0 = mT0 < trackStart? trackStart: mT0;
-      sampleCount start = track->TimeToLongSamples(t0);
+      auto start = track->TimeToLongSamples(t0);
 
       int analyzeSize = windowSize * numWindows;
       float * buffer;