do NOT use Glib::ustring unless you know that the contents are UTF-8 AND that you...

This fixes a Glib::ConvertError that occured when using Glib::ustring::operator<<
inside a compose operation. This implicitly uses Glib::locale_from_utf8(), and if the string
is not legal UTF-8, an exception will be thrown.

std::string should be used EVERYWHERE unless glyph-by-glyph iteration is required. This is
very rare in the Ardour codebase, so you really shouldn't see Glib::ustring anywhere. The
main exception is handling user-input for a few specific cases.