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/common/portrait.xml b/gui/theme/common/portrait.xml
index 80e358b..7e30be4 100644
--- a/gui/theme/common/portrait.xml
+++ b/gui/theme/common/portrait.xml
@@ -3779,6 +3779,11 @@
 				<text>{@swipe_to_unlock=Swipe to Unlock}</text>
 				<action function="overlay"/>
 			</slider>
+
+			<action>
+				<touch key="power"/>
+				<action function="togglebacklight"/>
+			</action>
 		</page>
 
 		<page name="filemanagerlist">