iOS: remove unneeded bool block

We used m_ignoreKeyboardChanges to ensure that we didn't
scroll the screen up and down and emit signals when
we applied the resign/become first responder work-around
to update input traits (input method hints).

A better way is to instead delay processing the
keyboardDidChangeFrame notification. We might still
get try to scroll several times during a single event
loop callout, but since we now start the scroll from the
bounds of the display layer, this would effectively end
up as a no-op.

Change-Id: If39284053a1bd3b4f31343b0d32c0366c093869b
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