[FEATURE] Replace current loading screen when loading multiple instances of app (macOS)

[bkunda]

This is a priority task for 4.1.

Note that it only partly solves the problem of multiple app instances currently experienced on macOS (see #12647 for more info).

We intend to deal with the more complex problem of handling multiple app instances in a later version of MuseScore 4 (most likely 4.2).

For now, we intend to replace repeated displays of this loading screen with something that does not suggest the app is actually being opened again and again:

(Note: this screen will still be displayed on open when no other instances of the app are already open).

[Tantacrul]

I'd suggest some kind of "Opening score" progress dialog of some kind.

[jessjwilliamson]

Designs are in 'MS4 LOADING SCREEN' in the Master project.

[DmitryArefiev]

@bkunda This is only for MacOS? The only difference between master and this change is adding a progress dialog in the next instance of MS, right?

[DmitryArefiev]

Implemented in #17509