Validator: added initializer for attributes

Similar to the tags, the attributes can be initialized. Missing tags are
automatically added. The declaration syntax is currently a bit annoying,
as the tags, that belong to an attribute, either have to be declared
explicitly or a pointer to the tag declaration must be given, but then
only concurrent tags are possible.
Support for global attributes is likewise missing; it must be ensured,
that (tag_n != 0) && (tags != NULL). Otherwise validator will be
inconsistent and there might be a bug.