Tagged-PDF: Parsing of StructElem standard types and attributes

Parse attributes and standard types of StructElem nodes of the
document structure tree. Type name aliases are resolved via the
RoleMap (and cycles detected). Both standard attributes and user
properties are mapped to instances of the Attribute class.
Attributes are parsed both via ClassMap references and directly
referenced from the StructElem objects.

https://bugs.freedesktop.org/show_bug.cgi?id=64815