text: Don't accept new line characters in single line mode

Modified Return key presses don't trigger ::activate so we would end
up adding an unprintable character to a single paragraph mode pango
layout which renders it as a box.

https://bugzilla.gnome.org/show_bug.cgi?id=623344

clutter/clutter-text.c

@@ -2074,7 +2074,7 @@ clutter_text_key_press (ClutterActor    *actor,
       if (key_unichar == '\r')
         key_unichar = '\n';
 
-      if (key_unichar == '\n' ||
+      if ((key_unichar == '\n' && !priv->single_line_mode) ||
           (g_unichar_validate (key_unichar) &&
            !g_unichar_iscntrl (key_unichar)))
         {