[Feature request] Score Editor

### Enhancement Summary

I would like to have an option to view, create and/or edit music using a score editor of LMMS (no need to import / export a midi file).

This is possible for example if you use Mixcraft or other software:

https://acoustica.com/mixcraft-10-manual/midi-editors-score-editor

https://archive.steinberg.help/nuendo_score/v12/en/cubase_nuendo_score/topics/score_editor_window/score_editor_window_r.html

### Implementation Details / Mockup

Allow multiple tabs: So you can switch between score and non-score editing options.

Example of Mixcraft:
https://youtu.be/94_oD-sT5tE?feature=shared&t=77
https://youtu.be/vQyO_k24LhA?feature=shared&t=83

### Please search the issue tracker for existing feature requests before submitting your own.

- [x] I have searched all existing issues and confirmed that this is not a duplicate.

The issue names no files, tests, or entry points; it describes a broad score-editor feature for LMMS and links to Mixcraft and Steinberg examples. Start by surveying the existing MIDI and non-score editing interfaces and their architecture. Done would require an agreed scope and design for viewing, creating, editing, and switching between score and other editing modes.

Written by the indexing model from the issue text.