gui: Actions: Toggle backlight on power key

Create GUIAction to handle KEY_POWER with a screen backlight toggle.

Change-Id: Iad0a7923b4a776e0336722db74d6fc46cd0107a2
diff --git a/gui/gui.cpp b/gui/gui.cpp
index 17ed108..3f1312f 100644
--- a/gui/gui.cpp
+++ b/gui/gui.cpp
@@ -205,7 +205,9 @@
 		break;
 	}
 
-	blankTimer.resetTimerAndUnblank();
+	if (ev.code != KEY_POWER && ev.code > KEY_RESERVED)
+		blankTimer.resetTimerAndUnblank();
+
 	return true;  // we got an event, so there might be more in the queue
 }