Fix #24630: Failure to load MuseSampler patches after instrument change

[mathesoncalum]

Resolves: #24630

In our current system, every Drumset we create starts off as an MS Basic patch and only becomes a MuseSampler patch once a mixer channel has been created for it (see usage of shouldLoadDrumset in PlaybackController). Due to the fact that we don't create new channels for identical instruments in a given part, the knock-on effect is that the new drumset won't have a chance to load a MuseSampler patch.

This PR does a couple of things:

1. If we try to change to a Drumset that already exists in the given part, we'll copy the patch from the existing Drumset
2. Changes to a Drumset (e.g. notehead changes) are copied for all identical Drumsets in a given part.

In an ideal world, we could avoid this "manual syncing" by having Instruments share pointers to the same Drumset. Doing so would require an extensive overhaul of much of our Instrument/Drumset logic, so we'll save this for another time.