Implement "take a screenshot" feature

* Like in android - press power+volume down, screenshots are saved
  in /sdcard/Pictures/Screenshots (if /sdcard is mounted) or /tmp

Change-Id: Iaefa15b11a1d5fdfac57d77388db1621f378a8d4
Signed-off-by: Vojtech Bocek <vbocek@gmail.com>
diff --git a/gui/devices/1920x1200/res/ui.xml b/gui/devices/1920x1200/res/ui.xml
index 2622a88..0841c96 100644
--- a/gui/devices/1920x1200/res/ui.xml
+++ b/gui/devices/1920x1200/res/ui.xml
@@ -277,6 +277,11 @@
 				<touch key="power" />
 				<action function="overlay">lock</action>
 			</object>
+
+			<object type="action">
+				<touch key="power+voldown" />
+				<action function="screenshot" />
+			</object>
 		</template>
 
 		<template name="progress_bar">
@@ -4159,6 +4164,11 @@
 				<action function="overlay">lock</action>
 			</object>
 
+			<object type="action">
+				<touch key="power+voldown" />
+				<action function="screenshot" />
+			</object>
+
 			<object type="template" name="keyboardtemplate" />
 
 			<object type="action">