Improve score audio by allowing custom releases of soundfont during playback

Initial post: https://musescore.org/en/node/281283
A custom release is an additional release sample that is tagged on to the initial attach-sustain sample to give a more realistic note ending. Currently, Musescore just cuts off the note by attenuation at the end of its value during playback, giving a rather synthetic effect. This misses the transient harmonics that could occur during the real release of an acoustic instrument. Interestingly, custom releases are played within the piano keyboard and when a single note is clicked. Please consider improving the score audio by allowing custom releases during playback as well.

See the following polyphone tutorial for audio and soundfont examples of "classical" vs custom releases: https://www.polyphone-soundfonts.com/en/documentation/tutorials/using-c…