Tagged-PDF: Modify pdfinfo to show the document structure
glib: Private function _poppler_link_mapping_new_from_annot_link()

Move the code that creates a PopplerLinkMapping to its own function, and add
it as private API. This will avoid duplication of the code when creating a
PopplerLinkMapping from a PopplerStructureElement.
glib: Private function _poppler_link_mapping_new_from_form_field()

Move the code that creates a PopplerFormFieldMapping to its own function,
and add it as private API. This will avoid duplication of the code when
creation a PopplerFormFieldMapping from a PopplerStructureElement.
glib: Accessors for form fields of structure elements

Implements two methods which can be used on PopplerStructureElement objects
of type POPPLER_STRUCTURE_ELEMENT_FORM:

- poppler_structure_element_get_form_field()
- poppler_structure_element_get_form_field_mapping()

The only difference is that the later returns a PopplerFormFieldMapping,
which itself contains the PopplerFormField, plus the rectangle covered by
the form widget in the page.
glib: Accessors for document structure references

Implements functions to work with PopplerStructureElement objects which
are references to other entities in the document. Whether an element is
a reference can be checked with poppler_structure_element_is_reference(),
and poppler_structure_element_get_reference_type() returns the type of
object referenced. For POPPLER_STRUCTURE_REFERENCE_LINK references, a
PopplerAction (or PopplerLinkMapping) can be retrieved using the
poppler_structure_element_get_reference_link_action() or
poppler_structure_element_get_reference_link_mapping() methods.
glib: Implement accessors for element attributes

Implement inspecting the standard attributes of PopplerStructureElement
objects.
glib: Fixes in the API reference documentation

- Remove the references to non-existent methods for PopplerStructureElement,
  and point to PopplerStructureElementIter instead to point out how to
  obtain the structure tree.
- Remove a non-existent include in poppler-docs.sgml
- Add POPPLER_TYPE_TEXT_SPAN to the list of private symbols.