Qt backend for ofSystemUtils

Continuing my work on ofSystemUtils, I’ve quickly made a Qt backend for it. It is still bit rough, the build system especially needs some attention as it is merely a hack now. But at least it seems to work, which is more than can be said about the GTK code (on my system).

You can see my code at github here: https://github.com/zokier/openFrameworks/compare/feature-ofSystemUtils_cleanup...feature-ofSystemUtils-Qt

This branch builds upon my previous branch where I splitted ofSystemUtils into pieces.