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.