Without setting firstResponder to 0 upon destruction, the current retain
count would never reach zero after the event was used. The result being
that QIOSTextResponder was seldom destroyed, which would also affect its
inputView etc which would also be kept alive.

Change-Id: Ia88e6a9d8764e7e9532487153e5e81a7ad0f9741
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>