Glib: Replace StringView with StdStringView and UStringView
Glib: Replace StringView with StdStringView and UStringView

StdStringView and UStringView force users to be explicit (by using c_str())
when using a std::string where a Glib::ustring is recommended or vice versa.
See issue #64

GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm