Thank you Coderus, and thank you Morpog for your script.

@Coderus:
I suppose that powermenu doesn't consume extra battery (since the system is already always waiting for signals of power button), does it?

@Morpog:
Before reading about the script, I selected Flashlight as the app for double click on power button. Now I can remove that app.
I tried to improve the script but then I see that /sys/kernel/debug/flash_adp1650/mode cannot be read. When launched from his icon, I see it takes some time to disapear from openned apps. I suppose that Jolla UI has a default minimal time wait for apps that cannot be changed. Changing it temporally from the script could be great.