iOS: Be more thorough when looking for current first responder

The technique of sending an action does not always end up at the actual
first responder, but it will end up in a responder in the responder
chain of the first responder, so we continue searching the subviews
recursively until we find the real first-responder.

Change-Id: I6abc9bc18eb127fa4b317cd308783c0ecfcd670a
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>