Fix #27025: Ensure excerpt solo-mute states are initialised properly for pre-4.4.0 scores

[mathesoncalum]

Resolves: #27025

Caused by #25021 and #19433

The main culprit is onTrackNewlyAdded - the idea of this method is to mute instruments newly added via the Layout panel in all excerpts (part scores) except for the current one*. Part of the problem is that, when loading pre-4.4.0 scores with existing parts, onTrackNewlyAdded gets a call and mutes the given track in all excerpts.

To avoid this we first need to initialise all excerpt solo mute states when loading pre-4.4.0 scores and be sure not to override that state when calling onTrackNewlyAdded by checking trackSoloMuteStateExists.

*#17250 is a known limitation related to this.

[DmitryArefiev]

@mathesoncalum Tested on Win10. #27025 looks fixed.

1. I only found an issue with 4.3.2 scores. If some instruments were unhidden and then hidden again in the part , they will not be muted in Mixer (in that part) when opened in PR's build (or Master build):

bandicam.2025-06-24.19-22-42-549.mp4

2. Also, I had some crashes but not sure if it's a regression comparing with master - crash.zip

[DmitryArefiev]

Retested #27025 and #28451 (comment) on Win10, Mac13.7.2. FIXED