deskutils/fet: Update to 5.39.0

• Add "gl" to USES as using USE_GL alone is deprecated.

Notable changes since 5.37.2:

• Updated to Qt 5.13.0
• New constraint types:
  • Activity tags not overlapping
  • Min gaps between an ordered pair of activity tags for a students set / all students / a teacher / all teachers
• Additional example files
• Translation updates
• Various crash fixes and code cleanups

PR: 239597
Submitted by: Zsolt Udvari <uzsolt@uzsolt.hu> (maintainer)