Do not load icons automatically everywhere

KatzeNet won't attempt to load icons that are not
cached unless the caller provides an icon callback.
For the moment, this means much less loading
for items that are not actually visible, but for
visiting pages or looking at bookmark menus.

katze/katze-net.c

@@ -531,7 +531,9 @@ katze_net_load_icon (KatzeNet*      net,
         }
         else if (g_file_test (icon_file, G_FILE_TEST_EXISTS))
             pixbuf = gdk_pixbuf_new_from_file (icon_file, NULL);
-        else
+        /* If the called doesn't provide an icon callback,
+           we assume there is no interest in loading an un-cached icon. */
+        else if (icon_cb)
         {
             priv = g_new0 (KatzeNetIconPriv, 1);
             priv->net = net;