Tablature: "Add Note" command needed (e.g. to allow creation of grace note chords)

One command missing from the MuseScore UI is the ability to select an existing fretmark in tablature and add another fretmark on a free string in the same position. This is mainly to allow the user to create grace note chords, but could be useful generally. The simplest option might be to implement "Add fretmark above" / "Add fretmark below" commands:

• The user selects an existing fretmark, and applies "Add fretmark above". Result: A ZERO fretmark is added on the next available string above the selected one.
• The user selects an existing fretmark, and applies "Add fretmark below". Result: A ZERO fretmark is added on the next available string below the selected one.