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/objects.hpp b/gui/objects.hpp
index f1c9c49..28ed29c 100644
--- a/gui/objects.hpp
+++ b/gui/objects.hpp
@@ -363,6 +363,7 @@
 	int checkpartitionlifetimewrites(std::string arg);
 	int mountsystemtoggle(std::string arg);
 	int setlanguage(std::string arg);
+	int togglebacklight(std::string arg);
 	int twcmd(std::string arg);
 	int setbootslot(std::string arg);
 	int installapp(std::string arg);