WIP: GString to ustring refactor

I'm working on replacing the c-construct GString with Glib::ustring. I could use some feedback on how to handle the difference between "g_strdup_printf" and "Glib::ustrin::compose" (%s, %d... vs %1, %2....). For the one case where I handled so far, I modified the ".po" files, but I'm realy not sure I'm supposed to do that. As I'm modifying a format string, all .po files with a translation of that line would need to get updated, or the translation for that line breaks.