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/os/add-ons/graphics/Accelerant.h

@@ -52,6 +52,8 @@ enum {
 	B_GET_PREFERRED_DISPLAY_MODE,		/* optional */
 	B_GET_MONITOR_INFO,					/* optional */
 	B_GET_EDID_INFO,					/* optional */
+	B_SET_BRIGHTNESS,                   /* optional */
+	B_GET_BRIGHTNESS,                   /* optional */
 
 	/* cursor managment */
 	B_MOVE_CURSOR = 0x200,				/* optional */
@@ -305,6 +307,9 @@ typedef status_t (*set_dpms_mode)(uint32 dpms_flags);
 typedef status_t (*get_preferred_display_mode)(display_mode* preferredMode);
 typedef status_t (*get_monitor_info)(monitor_info* info);
 typedef status_t (*get_edid_info)(void* info, uint32 size, uint32* _version);
+typedef status_t (*set_brightness)(float brightness);
+typedef status_t (*get_brightness)(float* brightness);
+
 typedef sem_id (*accelerant_retrace_semaphore)(void);
 
 typedef status_t (*set_cursor_shape)(uint16 width, uint16 height,

headers/os/interface/Screen.h

@@ -82,6 +82,9 @@ public:
 			uint32				DPMSState();
 			uint32				DPMSCapabilites();
 
+			status_t			GetBrightness(float*);
+			status_t			SetBrightness(float);
+
 private:
 	// Forbidden and deprecated methods
 								BScreen(const BScreen& other);