Instrument release time in milliseconds (#7217)

Make instruments report their release time in milliseconds so that it becomes independent of the sample rate and sounds the same at any sample rate.

Technically this is done by removing the virtual keyword from `desiredReleaseFrames` so that it cannot be overridden anymore. The method now only serves to compute the number of frames from the given release time in milliseconds.

A new virtual method `desiredReleaseTimeMs` is added which instruments can override. The default returns 0 ms just like the default implementation previously returned 0 frames.

The method `computeReleaseTimeMsByFrameCount` is added for instruments that still use a hard coded release in frames. As of now this is only `SidInstrument`.

Add the helper method `getSampleRate` to `Instrument`.

Adjust several instruments to report their release times in milliseconds. The times are computed by taking the release in frames and assuming a sample rate of 44.1 kHz. In most cases the times are rounded to a "nice" next value, e.g.:
*  64 frames -> 1.5 ms (66 frames)
* 128 frames -> 3.0 ms (132 frames)
* 512 frames -> 12. ms (529 frames)
* 1000 frames -> 23 ms (1014 samples)

In parentheses the number of frames are shown which result from the rounded number of milliseconds when converted back assuming a sample rate of 44.1 kHz. The difference should not be noticeable in existing projects.

Remove the overrides for instruments that return the same value as the base class `Instrument` anyway. These are:
* GigPlayer
* Lb302
* Sf2Player

For `MonstroInstrument` the implementation is adjusted to behave in a very similar way. First the maximum of the envelope release times is computed. These are already available in milliseconds. Then the maximum of that value and 1.5 ms is taken and returned as the result.

src/core/Instrument.cpp

@@ -195,8 +195,17 @@ void Instrument::applyRelease( sampleFrame * buf, const NotePlayHandle * _n )
 	}
 }
 
+float Instrument::computeReleaseTimeMsByFrameCount(f_cnt_t frames) const
+{
+	return frames / getSampleRate() * 1000.;
+}
 
 
+sample_rate_t Instrument::getSampleRate() const
+{
+	return Engine::audioEngine()->processingSampleRate();
+}
+
 
 QString Instrument::fullDisplayName() const
 {