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.

https://bugs.freedesktop.org/show_bug.cgi?id=75323
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.

https://bugs.freedesktop.org/show_bug.cgi?id=74753
Tagged-PDF: Fix parsing of attached element attributes

The wrong object was used as attribute dictionary, which caused
a segmentation fault when parsing PDF files in which the structure
elements included attributes attached to them. This patch fixes the
issue.

Thanks to Joanmarie Diggs <jdiggs@igalia.com> for helping in
debugging the problem.

https://bugs.freedesktop.org/show_bug.cgi?id=74805