Fix waveform concurrency issues. Possible fix for Bug #1383404

* Use const shared pointers for waveforms.
* Move all std::vector sizing operations to the Waveform constructor. No
  vector state is allowed to change after the constructor finishes.
* Clarify comments.
* Remove waveform mutex usage in WOverview variants.
* Use the track's waveform pointer to atomically swap the waveform
  rather than mutating the waveform in place.
* Remove unused code.
The original reason vrince said was to avoid the performance cost of bounds checking -- we write to the array via the data pointer (no cost) and read using operator[] (bounds checked in a QVector and bounds checked for std::vector in a Windows debug build).

We could switch to QVector and always access via the data pointer when we can prove the bounds are correct outside of the loop.