app: Fix parsing of YouTube categories
app: Fix parsing of YouTube categories

The GDataAPPCategories parser used to take in the GType of the
GDataCategory subclass to construct when parsing its category list; in
the port to JSON, it was accidentally changed to always construct
GDataCategory instances. This broke the YouTube code, which was hoping
to see GDataYouTubeCategory instances be constructed.

pwithnall/libgdata,GNOME/libgdata,pwithnall/libgdata