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/theme/portrait_mdpi/ui.xml b/gui/theme/portrait_mdpi/ui.xml
index 9a0adf3..8d424db 100644
--- a/gui/theme/portrait_mdpi/ui.xml
+++ b/gui/theme/portrait_mdpi/ui.xml
@@ -336,7 +336,7 @@
 
 			<action>
 				<touch key="power"/>
-				<action function="overlay">lock</action>
+				<action function="togglebacklight"/>
 			</action>
 
 			<action>