gui: Save pending track metadata before closing main window

[Krish2882005]

Closes #16191

This PR makes sure Mixxx saves pending track metadata to the database before the main window completely closes. Previously, this happened in the background after closing, which could potentially cause database or config trouble if the app was restarted too quickly.

What changed:

• Deactivates the track cache before the close event is accepted to trigger the metadata save.
• Displays a "Saving Metadata" message box so the user knows to wait while the app finishes up.

[ronso0]

While this works as a quick fix, my feeling is that it's not that easy.
What can happen until the same, now redundant call from TrackCollectionManager::~TrackCollectionManager()?
And what about the other cleanup tasks in GlobalTrackCache::destroyInstance(), implicitly called by CoreServices::finalize()?