Fix loading of some animation files

For some files KoXml::asQDomElement() works incorrectly.

Ideally, we should deprecate KoXmlReader and friends, but we
would need to do the following steps:

1. Benchmark and compare KoXmlReader vs QDomDocument. If the difference is not extremely drastic, then proceed to the next step.

2. Refactor out all the uses of KoXmlReader and KoXmlWriter.