ReportBuilder: Take a std::locale instead of a std::string locale name.

* glom/libglom/data_structure/glomconversions.[h|cc]:
  Remove the recently-added get_text_for_gda_value() overload
  that took a ustring locale ID.
* glom/libglom/report_builder.[h|cc]: Constructor: Take a
  std::locale instead of just a locale name, because we need the
  full locale (name) for use with get_text_for_gda_value().
  Use Utils::locale_simplify() to get the locale ID for use with
  translations.
  This means that we will ignore a temporarily-chosen translation,
  because we have no way to get the full locale name from the
  simpler currently-set locale ID, but that is not so bad.
* glom/frame_glom.cc:
* glom/mode_data/box_data_manyrecords.cc:
* tests/test_selfhosting_new_then_report.cc:
* tests/test_selfhosting_new_then_report_summary.cc: Adapted.