Tagged-PDF: Interpret the document structure

Picking from StructTreeRoot, recursively creates a tree of StructTreeNode
objects representing the structure of the document. The biggest missing
things are:

- Presenting more information in StructTreeNode, being the most
	important reading the structure and attribute dictionaries for
	the elements in the tree.
- Resolving marked-content identifiers which refer to information
	stored in page object streams.
- Creating a synthetic tree when the PDF is not tagged to use as
	fall-back.
Tagged-PDF: Modify pdfinfo to show the document structure
Tagged-PDF: GLib poppler-structure WIP