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_form_get_field()
- poppler_page_get_form_field_mapping_for_structure_element()

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: Methods to obtain link information from structure elements

Implements functions to work with PopplerStructureElement objects of
type POPPLER_STRUCTURE_REFERENCE_LINK:

- poppler_structure_element_link_get_action() retrieves the PopplerAction
  associated with the link.
- poppler_page_get_link_mapping_for_structure_element() obtains the
  PopplerLinkMapping associated with the link referenced by a structure
  element.
glib: Show structure element standard attributes in demo

In poppler-glib-demo, display the values of the standard structure element
attributes. This also makes the grid view scrollable to make it possible to
see all the attributes when the window is small, and the horizontal GtkBox
is changed to a GtkPane, to make it possible to resize the tree in the left
column.