Fix comments from code review.

1) #include gui.h in effects.c.

2) Copy/paste error in xml_settings for UNOVIBE_CHORUS_VIBRATO.

3) Use a gboolean instead of a guint.

4) Remove a warning that fires under normal circumstances on devices
   without LFO's.

gui.c

@@ -858,8 +858,6 @@ create_modifier_group (guint pos, guint id)
      */
     g_tree_steal(widget_tree, key);
     if (!list) {
-        g_warning("No widget tree entry for position %d id %d!\n",
-                   pos, id);
         return;
     }