Add a simple label for "root" note and selected key/note range

Nobody has claimed this yet.

### Enhancement Summary

#5868 added also a feature to select a key/note range by dragging an arrow from the left and/or the right side of the piano inside the instrument window.

The idea of this enhancement is to add a simple informational label for displaying the "root" note and the selected key/note range. Somehow similar as seen here in the piano roll limiter of FL Studio:


Taken from here: https://www.image-line.com/fl-studio-learning/fl-studio-online-manual/html/pianoroll_limit.htm

#### Justification

The key/note range is not really visible in the first place and can only be discovered by scrolling to the begin or the end of the piano view which many users may not do without already knowing about the feature. At least this is the state of the master branch some weeks ago.


The discussion in #6210 also showed that this feature may not be as obvious as it should be, because this already implemented feature would e.g. allow the assignment of different notes on channel 10 to use a drum pad. Of course this would not be a really convenient way to set up such a midi channel and key/note assignment - for this a different UI solution may be needed - but it would give an indirect hint to users that a possibility to modify this key/note range is at least likely.

And the additional "root" note display may allow a better visual indication if it was set to a black or white key which is sometimes difficult to see just by the rectangle marker as seen here:


#### Mockup


A slight increase in height of the instrument window may be needed though.

Start by reviewing the key/note range selection feature from issue #5868 and the discussion in #6210, then inspect the piano UI in the instrument window. Compare the current display with the supplied mockup. Done means the root note and selected key/note range are visibly labeled without requiring users to scroll to either edge.

Written by the indexing model from the issue text.