hb_system_sleep: allow/prevent sleep from the UI rather than libhb.

The code that sets/unsets the "power assertions" is still in libhb.

Patch by Jérôme Lacube. Thanks!



git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5336 b64f7644-9d1e-0410-96f1-a4d463321fa5