gio: Add source tags to various GTasks constructed in GLib

This makes them easier to identify when debugging and profiling.

This patch was somewhat less than interesting to write.

https://bugzilla.gnome.org/show_bug.cgi?id=767765

gio/gdummyproxyresolver.c

@@ -95,6 +95,7 @@ g_dummy_proxy_resolver_lookup_async (GProxyResolver      *resolver,
   gchar **proxies;
 
   task = g_task_new (resolver, cancellable, callback, user_data);
+  g_task_set_source_tag (task, g_dummy_proxy_resolver_lookup_async);
 
   proxies = g_dummy_proxy_resolver_lookup (resolver, uri, cancellable, &error);
   if (proxies)