German translation

Still missing a few messages:

* `OPF_008=Handler binding for core Media-type '%1$s' is not allowed.`
* `OPF_009=The media-type %1$s has already been assigned handler '%2$s'.`
* `OPF_046=Scripted property is not set on mediaType handler.`
* `RSC_012=Fragment identifier is not defined.`
* `RSC_013=Fragment identifier is used in a reference to a stylesheet resource.`
* `RSC_014=Fragment identifier defines an incompatible resource type.`
* `RSC_015=A fragment identifier is required for svg use tag references.`

**Übersetzungsvorschläge sind herzlich Willkommen!** :smiley: 

@santoch: Can you ask Holm to revise the translation?

I hereby also ask my colleagues @duenckel and @andreaskaemmerle whether they're in the mood to revise the translation.