iOS: implement 'close keyboard' gesture

Before this patch there were no way for the user to hide
the keyboard on iPhone for multi-line edit fields unless
the app had a separate button added for it. And even that
would be problematic since we scroll the screen (and
perhaps the button) to track the cursor.

This patch implements a gesture that resembles the 'hide
keyboard' gesture that UIScrollView implements on iOS 7.

Change-Id: I1c0d9c88ff1f5430587a49591f165b9708e5dc60