cleanup: use taxonomy_set_category() function

Instead of manipulating the taxonomy structures directly, use the
taxonomy_set_category() function. This improves encapsulation and
gives us the possibility to improve the taxonomy data structures.

This concerns three places:
1) git parser
2) XML parser
3) reverse geo-lookup

This improves the XML parser code slightly: The parser assumes that
the value-attribute comes last (after origin and category). While
it still does that, it now at least generates a warning if it encounters
a value-attribute without origin- or category-attribute.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/parse.h

@@ -52,6 +52,7 @@ struct parser_state {
 	struct sample *cur_sample;		/* non-owning */
 	struct picture cur_picture;		/* owning */
 	char *country, *city;			/* owning */
+	int taxonomy_category, taxonomy_origin;
 
 	bool in_settings;
 	bool in_userid;