Add idle dim switch

[danirabbit]

Fixes #168

[jeremypw]

The UI is good for changing the Gnome setting "idle-dim". However it is not really usable as it is in my opinion because both dimming and display off seem to be using the same session setting "idle-delay". So changing both happen at the same time and the dimming is never seen (as far as I can tell).

[zeebok]

The UI is good for changing the Gnome setting "idle-dim". However it is not really usable as it is in my opinion because both dimming and display off seem to be using the same session setting "idle-delay". So changing both happen at the same time and the dimming is never seen (as far as I can tell).

Oh yeah, the gnome docs use idle-delay in their explanation of how to do idle dimming. Strangely I don't see anything about idle suspending?