iOS: Make QIOSTextInputResponder a proper first-responder during text input

Instead of faking it, by returning YES for isFirstResponder, which caused
issues when iOS would try to dismiss the keyboard by resigning the true
first-responder.

Change-Id: I816c4cf9c699d72995ce7968e1f1a4aa9c9c167e
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>