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/action.cpp b/gui/action.cpp
index 7d34da8..ef110bc 100644
--- a/gui/action.cpp
+++ b/gui/action.cpp
@@ -198,6 +198,7 @@
 		ADD_ACTION(mountsystemtoggle);
 		ADD_ACTION(setlanguage);
 		ADD_ACTION(checkforapp);
+		ADD_ACTION(togglebacklight);
 
 		// remember actions that run in the caller thread
 		for (mapFunc::const_iterator it = mf.begin(); it != mf.end(); ++it)
@@ -1865,6 +1866,12 @@
 	return 0;
 }
 
+int GUIAction::togglebacklight(std::string arg __unused)
+{
+	blankTimer.toggleBlank();
+	return 0;
+}
+
 int GUIAction::setbootslot(std::string arg)
 {
 	operation_start("Set Boot Slot");