[libdazzle] actions: add fallback muxer key

[Christian Hergert <chergert src gnome org>]

commit 73b236857aae6624326dcb220a9002c9a48fb974
Author: Christian Hergert <chergert redhat com>
Date:   Thu Jul 6 15:57:24 2017 -0700

    actions: add fallback muxer key
    
    Some simple situations can use a default muxer key, so at least
    allow that to work.

 src/util/dzl-gtk.c |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)
---
diff --git a/src/util/dzl-gtk.c b/src/util/dzl-gtk.c
index 91d2b78..ff49b7f 100644
--- a/src/util/dzl-gtk.c
+++ b/src/util/dzl-gtk.c
@@ -372,7 +372,7 @@ dzl_gtk_widget_action_set (GtkWidget   *widget,
  * dzl_gtk_widget_mux_action_groups:
  * @widget: a #GtkWidget
  * @from_widget: A #GtkWidget containing the groups to copy
- * @mux_key: a unique key to represent the muxing
+ * @mux_key: (nullable): a unique key to represent the muxing
  *
  * This function will find all of the actions on @from_widget in various
  * groups and add them to @widget. As this just copies the action groups
@@ -392,7 +392,10 @@ dzl_gtk_widget_mux_action_groups (GtkWidget   *widget,
 
   g_return_if_fail (GTK_IS_WIDGET (widget));
   g_return_if_fail (!from_widget || GTK_IS_WIDGET (from_widget));
-  g_return_if_fail (mux_key != NULL);
+  g_return_if_fail (widget != from_widget);
+
+  if (mux_key == NULL)
+    mux_key = "DZL_GTK_MUX_ACTIONS";
 
   /*
    * First check to see if there are any old action groups that