deskutils/fet: update to 6.15.0

ChangeLog: https://lalescu.ro/liviu/fet/news.html

• Added the possibility of saving the history to the hard disk, next to the saved .fet data file (suggested by uni_instructor and ngoctp29121982). This option is by default disabled and must be enabled from the settings.
• Minor improvements in the memory consumption (the comments of the history are now stored archived).
• Code cleanup.

PR: 275890
Reported by: uzsolt@uzsolt.hu (maintainer)