In mt32emu_qt:
- Added support for using different sample rates in audio drivers using sample rate conversion.
  It is still preferable to use generic sample rate of MT-32 compatible units 32000 Hz and sample rate conversion
  should be used if a different sample rate is necessary (e.g. audio system doesn't support 32000 Hz) or deliberate.
  For best performance, libsoxr is supported. Libsamplerate is also supported as it is widely available.
  As a worst case, the integrated linear interpolator can be used (it's fast but the quality...).
In mt32emu_qt:
- PortAudio driver now displays corresponding error description if it fails to open
In mt32emu_qt:
- QSynth is now properly closed if SynthRoute fails to start an AudioStream
In mt32emu_qt:
- Fixed output volume when using float samples to correspond the integer samples mode