Tagged-PDF: Expose the structure tree in poppler-glib

Implements two new GObject classes, which build upon StructTreeRoot
and StructElement to expose the document structure of tagged PDFs
in the GLib binding:

- PopplerStructure wraps StrucTreeRoot, and contains additional utility
  methods to search for elements in the structure tree.
- PopplerStructureElement wraps StructElement, and does the heavy lifting
	of exposing data in GLib-friendly data types.

For standard attributes, to avoid cluttering the class with fmethods, a
single poppler_structure_element_get_attribute() method is implemented,
which returns either NULL (for undefined attributes) or a GVariant
containing a sensible representation of the value.
Tagged-PDF: Pane in poppler-glib demo showing the structure

Adds a new pane in poppler-glib-demo showing the structure for Tagged-PDF
documents. It also serves as an example on how to to use the API for
PopplerStructure and PopplerStructureElement.