Support for Playback of Ghost Notes

When writing drum scores ghost notes can be made visible in the score using bracket, however there is no functionality associated to them concerning playback. It is of course possible to select notes one by one (with or without bracket) and offset their velocity in the navigator.

The request is to have the possibility to associate a velocity offset with the bracket in general, which is
- configurable in the range of +/- 127 (for flexibility, even if for ghost notes only negative values make sense)
- stored per score.
- initialized for each new score with a default value of 0

Im not an expert enough to judge if the bracket on note is used for other purposes than ghost notes, however the feature becomes neutralized by a velocity offset value "0".