gtk: fix entering of Option-foo symbols on the Mac

define GTK_NO_TEXT_INPUT_MOD_MASK in gtkprivate.h
and use it in the IM contexts, so Option-foo is no longer
filtered away.

gtk/gtkimmulticontext.c

@@ -350,7 +350,7 @@ gtk_im_multicontext_filter_keypress (GtkIMContext *context,
   if (slave)
     return gtk_im_context_filter_keypress (slave, event);
   else if (event->type == GDK_KEY_PRESS &&
-           (event->state & (GDK_MOD1_MASK | GDK_CONTROL_MASK)) == 0)
+           (event->state & GTK_NO_TEXT_INPUT_MOD_MASK) == 0)
     {
       gunichar ch;