Move notes inside a selection in piano-roll

Nobody has claimed this yet.

This would be a very useful method, when you are working with chords:
Quite often i have chords-**selections** of 5 or more notes.

These are slightly different from bar to bar - eg, a few notes is changed.

If i then want to alter _one_ of the notes in this selected chord,
i have to 'un-select' the whole selection, either
- by LMC, that will create a new note in the pos i like, and then delete the surplus note.
At the same time, the selection is de-selected. Eg -If i still want to work with this selection, I now have to re-select

or
- RMC the note i like to change. That will delete _that_ note, and i can then place a new note in the new pos. At the same time, the selection is de-selected, and re-select is again necessary:

There are several drawbacks in both these workflows:
Even though the 'job' is done,either way, it involves at least one unnecessary deletion of a note, _and_ an unwanted de-selection of the selected notes, with a re-selection as result.

What would be optimal, would be a key-press-combo that would exclude the mouse-klicked note, _from_ current selection, and allow me to move _that_ single note. At mouse-release, the note should again be inserted in the original selection.
This whole selection is now easy to copy to the next bar

It would greatly enhance working with larger chords-selections.

((On top of this we also need #640 for working smoothly with chords))

No source file, test, or entry point is named. Start by reproducing the piano-roll selection and mouse-editing workflows described here, then inspect the existing selection and note-movement behavior; done means one selected note can be moved while the remaining selection is preserved and the moved note returns to the selection, while considering the related #640 work.

Written by the indexing model from the issue text.