use find_executable instead of hardcoding the path to xrandr

Hi,
i am using your xprofile on OpenBSD for a few hours and it works great so far! The only thing i had to change was the hardcoded path to xrandr. On OpenBSD xrandr is installed under /usr/X11R6/bin. This pr uses find_executable instead of hardcoding the path.

I am not really an python programmer so if there's a better way to find the path to an executable, just leave a comment and i will send a new pr.

Cheers,
Fabian