GtkActionHelper: add some debugging output

Introduce a new debug category "actions" and write some messages from
GtkActionHelper about if we can find the actions or not.

We will probably soon want to add some similar messages to
GtkMenuTrackerItem.

https://bugzilla.gnome.org/show_bug.cgi?id=733965

gtk/gtkdebug.h

@@ -53,7 +53,8 @@ typedef enum {
   GTK_DEBUG_PIXEL_CACHE     = 1 << 15,
   GTK_DEBUG_NO_PIXEL_CACHE  = 1 << 16,
   GTK_DEBUG_INTERACTIVE     = 1 << 17,
-  GTK_DEBUG_TOUCHSCREEN     = 1 << 18
+  GTK_DEBUG_TOUCHSCREEN     = 1 << 18,
+  GTK_DEBUG_ACTIONS         = 1 << 19
 } GtkDebugFlag;
 
 #ifdef G_ENABLE_DEBUG