* glom/mode_design/translation/window_translations.[h|cc]:
load_from_document(), save_to_document(): Move the collecting of
translatable items to:
* glom/libglom/document/document.[h|cc]: get_translatable_items();
Also move the gettext/po import/export to:
* Makefile_libglom.am:
* glom/libglom/filelist.am:
* glom/libglom/translations_po.[h|cc]: write_translations_to_po_file()
and import_translations_from_po_file().

* Makefile_tests.am
* tests/translations_po/data/test.po:
* tests/translations_po/test_document_export_po.cc:
* tests/translations_po/test_document_import_po.cc: Add tests of the new
libglom functions.

* Makefile_glom.am
* glom/glom_export_po.cc: Added a new command-line tool that uses the
new libglom API.