Add initial avatar support

We change the code for dealing with contacts quite significantly,
adding a new class, CallsBestMatch, in order to encapsulate a lot of
the functionality that was in CallsCallRecordRow.

At present avatars are not rounded and there are no auto-generated
avatars for contacts with no avatar.  This work is awaiting discussion
of Apps_Issues#164.

src/util.h

@@ -97,6 +97,10 @@ G_BEGIN_DECLS
     }
 
 
+/** If the GObject object is non-NULL, unref it */
+void calls_object_unref (gpointer object);
+
+
 /** Find a particular pointer value in a GtkListStore */
 gboolean
 calls_list_store_find (GtkListStore *store,