Support for configuring screen backlight

Accelerant interface:
Introduce new hooks B_SET_BRIGHTNESS and B_GET_BRIGHTNESS. Brightness is
a float in the 0..1 range.

App_server:
Forward brightness things between BScreen and the accelerant.

intel_extreme:
Implement the hooks. Note that this only works for laptop panels, but
the driver will pretend to support it in other cases as well.

Screen preferences:
If the accelerant supports the B_GET_BRIGHTNESS hook, allow to set
brightness with a slider. Otherwise, the slidere is hidden and these
changes aren't visible.

headers/private/interface/PrivateScreen.h

@@ -82,6 +82,9 @@ public:
 			uint32				DPMSState();
 			uint32				DPMSCapabilites();
 
+			status_t			GetBrightness(float*);
+			status_t			SetBrightness(float);
+
 			void*				BaseAddress();
 			uint32				BytesPerRow();