Get rid of OFXMLFactory.

There are two reasons for this:

1.) This was one of the oldest classes. Back then when it was written,
    a lot was missing in the framework, so it had to be implemented by
    the class itself. This means there was code duplication, but fixing
    that wouldn't make sense due to the second reason.
2.) There is OFXMLElement now, which is much more flexible, much more
    fun to use and can do everything that OFXMLFactory could.