* glom/libglom/data_structure/tableinfo.[h|cc]: Add
get/set_title_singular(), using a member TranslatableItem, to hold
the singular form (such as Album instead of Albums) for use in the UI
for users.
* glom/libglom/document/document.cc: save_before(), load_after():
Load and save the singular table titles as a new node.
* glom/glom_document.dtd: Mentin the new node, though I don't think
we document translation nodes properly.
* glom/navigation/box_tables.[h|cc]: In developer mode, show the column
so the user can enter a singular title.