Make UI code easier to track

[kaixoo12]

This is something I've got planned for my next PR.

Currently the UI code is put directly in the code. This makes visualizing the code harder. Ideally we should have the UI declaration in separate files (.ui files), and only use it when necessary in the code.

This would also make it possible to add customizability to the UI in the future.

[pgilfernandez]

Yes, I agree there is work to do in this area, I would say UI should also move to QSS files to let other collaborators or users to create themes

[kaixoo12]

And in the long long term we should consider QML instead of QWidgets. It's what MuseScore and Audacity use and it looks very nice. But ofc that's not a priority now.

For now if someone can start making the transition to .ui and .qss, step by step and little by little it should be possible. At least for large files like mainwindow