Selected Music tracks disappear if they update

[RichardULZ]

Describe the bug
OpenMPT, like gbstudio, renames working files to .bak before writing an new file.
This short timespan where the music file is missing (renamed) can cause gbstudio to loose previously assigned music files, and requires you to re assign the track wherever it was used.

To Reproduce
Steps to reproduce the behavior:

1. Assign a music play event with a .mod
2. Rename the music .mod file
3. See the file selection disappear in music event.
4. Rename back to the original file name
5. Track remains empty in gbs event.

Expected behavior
Preferably the missing assigned tracks would turn red in gbs music event, and found again when a track is re added.

Else, a small time buffer before clearing tracks that have been removed.

Screenshots
If applicable, add screenshots to help explain your problem.

Platform (please complete the following information):

• OS: [ Windows 10 ]
• App version: [1.2.1 (Issue since 1.0?)]

Additional context
GBstudio's src/lib/project/WatchProject.js seems the likely place this happens, but perhaps the fix would be best in the music play event, to store the selected song name and check for updates when missing, else how would you tell if a removed song was used previously at all.

You can work around this by saving .mod outside the project, and replacing the music file in windows manually, or reopening an older version of the gbsproj. In most cases this is slower than reassigning the track in gbstudio, unless it was used more than once.

This is a similar issue to #387 just without the crash ;)