Disable screensaver

[hmeine]

During a presentation, both the screensaver and corresponding power management features should be disabled.

[hmeine]

For OS X, I found UpdateSystemActivity; possibly, there is a more modern IOKit-based approach, at least for IOS (from which frameworks often appear on OS X sooner or later, nowadays).
For Linux, I plan to use xdg-screensaver as found on StackOverflow, but using the real winid (from Qt) and not shell=True for subprocess.