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.
glib: Handle missing structure element types

Element Art (article); RB, RP, RB (Ruby text inner elements); and
WT, WP (Warichu inner elements) were not being handled. This adds
the corresponding handling in poppler-glib. Also, the "default" case
in the switch in poppler_structure_element_get_type() is removed, so
the compiler can emit warnings when enum values are not handled.