Tagged-PDF: Modify pdfinfo to show the document structure
Tagged-PDF: Implement the utils/pdfstructtohtml tool

This adds a new tool "utils/pdfstructtohtml" that uses the document
structure from tagged PDFs to generate an HTML document from it, trying
to preserve as much of the structure and some of the styling in the
output. This serves also as a demonstration of how to use the Attribute
and StructElement classes.
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.
Tagged-PDF: Heuristics in poppler-glib for data/layout table identification

Add functions in poppler-glib which, used on a PopplerStructureElement of
type POPPLER_STRUCTURE_TABLE, determine whether a table is used for layout
or contains actual data. The heuristic is quite simple so far:

- poppler_structure_element_is_data_table(): checks for table headings
  and that THead/TBody elements are present.
- poppler_structure_element_is_layout_table(): negates the result of
  the previous function.

The idea is that in the later case a different heuristic could be
implemented later on, and at some point tables could be "data tables",
"layout tables", or the heuristics can not tell for sure.