iOS: Allow virtual keyboard to be hidden by QInputMethod::hide()

Calling the function has the same effect as dismissing the keyboard using
the native keyboard dismiss button or the hide-keyboard gesture, and will
result in the QIOSTextInputResponder losing first-responder status and
the current focus object being cleared.

QtWidgets and other parts of Qt will try to hide the keyboard during
focus changes between widgets, which we already take care of when the
focus object changes, so we detect the situation and ignore it, by
requiring that the current focus object matches the one we've brought
up the text responder for.

Showing the virtual keyboard is still a no-op, as there is no way to
show the virtual keyboard without a focus-object.

Change-Id: Iefcb403c2b6d3da8a4df3fcd53bc1244ba9c4d23
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>