Fix crash with missing midi_settings

fActiveSoundFont wasn't initialized when there's no midi_settings file.
Don't allocate it on the heap, the object get implicitely initialized to
an emtpy string.
Tiny style fix.
Thanks to AnEvilYak for his consultation.

Fixes #13402.

Improved MIDI section of Media preferences

* Get rid of the "Actve Sound Font:" BStringView below the BListView.
Now the selected SoundFont is the active SF.
* If there's only one SF available, select it automatically.
* Added a BStringView below the list and only give feedback when needed:
- if no SF is installed
- if the user has to select a SF
* Double-clicking an entry opens its location in Tracker.
* Added node-monitoring to live-update the list of available SFs.
Note: Un/installing a package doesn't trigger a notification (related
to #9970 ?).

MediaPreferences: midi: added a BStringView with the currently active soundfont

Integrate MIDI settings with the Media preflet.
More or less working.

Integrate MIDI settings with the Media preflet.
More or less working.