Add option to display chord symbols as Roman numerals

[mbadrelzaman]

It would be helpful to have a toggleable option to display chord symbols using Roman numeral analysis based on the song’s key (e.g., Cmaj7 → Imaj7, Dm7 → ii7 in the key of C). This feature would support understanding harmonic function, learning music theory, and practicing transposition.

When the user activates Roman numeral view:

• The system prompts for the original key of the progression.
• A checkbox allows the user to choose between flat or sharp symbols for accidentals.
• Chords are then displayed as Roman numerals instead of standard chord names.
• Optionally, users could transpose the progression from the original key to a target key, using Roman numerals as the intermediate framework.

This feature would enhance JJazzLab’s usefulness as an educational and compositional tool.

[jjazzboss]

This is doable but needs some careful UI design...

Asking user to provide the original key simplifies things, but a song often uses several keys... We should give the possibility to select bars or chords before setting the key.

[jjazzboss]

Issue also linked to other user requests to get "the key of the song" or similar.

Reuse some code from the (old) "impro-visor" java software which had an interesting "roadmap" analysis ?
https://www.cs.hmc.edu/~keller/jazz/improvisor/Roadmap507.htm