glib: Implement accessors for element attributes

Implement inspecting the standard attributes of PopplerStructureElement
objects.

https://bugs.freedesktop.org/show_bug.cgi?id=64821
glib: Rename PopplerStructureScope as PopplerStructureTableScope

It's a table specific attribute
glib: Update the documentation symbols after the PopplerStructureScope rename
Rename getNumElements, getElement and appendElement as getNumChildren, getChild and appendChild

It's more consistent with other internal API and less confusing.