Show loudness % when B&B blocks are mouse-scrolled

Inside BB-editor we can change the loudness with mouse-scrolling on a step-editor block:

https://lmms.io/wiki/images/6/60/BeatBassline_step_editor1.0.0.png

This is audible -ofcause, but there is no exact read-out of actual value.
(Only in piano-roll, can we read-out the vol, as an exact value.)
It would make sense, that this read-out, was possible directly in BB-editor

Start by locating the BB-editor step-editor code and comparing its mouse-scroll loudness handling with the piano-roll's exact volume read-out. Done means the BB-editor shows the current loudness percentage while a block is adjusted by mouse scrolling.

Written by the indexing model from the issue text.