Use org.freedesktop.ScreenSaver API to inhibit screensaver

As discussed on ticket 4739.

I've implemented the "freedesktop"1 interface in GNOME, it will be available for GNOME 3.6 (the current version) and later2.

Please use this (the inhibit/uninhibit calls) to implement the screensaver inhibition in VLC.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information