Add when_screen_edge_panned for UIScreenEdgePanGestureRecognizer

`UIScreenEdgePanGestureRecognizer` is an iOS7 and newer recognizer for detecting a pan at defined edges of the screen.

I didn't add a `if Device.ios_version < "7.0"` assuming that a proper `uninitialized constant UIScreenEdgePanGestureRecognizer` would be raised in the case that it's called in a pre-iOS7 environment. Let me know if such a check is necessary, but I do not have any way to test behavior in iOS6 and older.