* glom/libglom/data_structure/translatable_item.[h|cc]:
get_translatable_item_type(): Make this const.
* glom/libglom/document/document.[h|cc]: get_translatable_items():
Now return a list of pairs, so each TranslatableItem also has a hint
for the msgtxt for the po (gettext) file, so that each can be
identified uniquely and so that the translator has some context.
* glom/libglom/translations_po.[h|cc]: Adapt.
* glom/mode_design/translation/window_translations.[h|cc]: Adapt,
showing the actual hint instead of just the item type.
* tests/test_document_load_translations.cc: Adapted.
* tests/translations_po/data/test.po: Reexported.