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: Add poppler_structure_element_is_grouping()

Implement a method to check whether a structure element is a grouping
element, wrapping the StructElement::isGrouping() method.