deskutils/fet: Update to 7.5.7

Remove conflicts and add entries to MOVED (see commit
ff38dac0b6f33a939cfa1cace4e4c083dbcf91cb; suggested by jhale).
Switch gl to opengl in USE_GL (because of Qt6, suggested by jhale).

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