[glibmm] (7 commits) ...Merge branch 'wip/dboles/ustring_format_fold_and_test' into 'master'

[Kjell Ahlstedt <kjellahl src gnome org>]

Summary of changes:

  fbe267d... ustring: Use C++17 fold expr. instead of recursion (*)
  a485da1... tests/glibmm_ustring_format: Test output, iomanips (*)
  f6e3298... ustring: Make 1 Stringify member const like others (*)
  c4de725... ustring: Avoid un(necessary|clear) ptr @ Stringify (*)
  3b3d656... ustring: Remove useless alternative Stringify ctor (*)
  25846ce... tests/glibmm_ustring_format: Fix non-English langs (*)
  410e03e... Merge branch 'wip/dboles/ustring_format_fold_and_test' into

(*) This commit already existed in another branch; no separate mail sent