Use non-deprecated sleep assertion on OSX

I don't think it will change anything but the assertion we used
is deprecated going forward so it was worth switching for that reason
alone.

src/power/PowerComponentMac.cpp

@@ -6,7 +6,7 @@ void PowerComponentMac::doDisableScreensaver()
   if (m_assertion == 0)
   {
     CFStringRef why = CFSTR("tv.plex.player");
-    IOPMAssertionCreateWithName(kIOPMAssertionTypeNoDisplaySleep,
+    IOPMAssertionCreateWithName(kIOPMAssertionTypePreventUserIdleDisplaySleep,
                                 kIOPMAssertionLevelOn,
                                 why,
                                 &m_assertion);