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.c

@@ -24,6 +24,17 @@
 
 #include "util.h"
 
+
+void
+calls_object_unref (gpointer object)
+{
+  if (object)
+    {
+      g_object_unref (object);
+    }
+}
+
+
 typedef struct
 {
   gpointer needle;