Support g_main_context_push_thread_default() in gio

GFile allows for the possibility that external implementations may not
support thread-default contexts yet, via
g_file_supports_thread_contexts(). GVolumeMonitor is not yet
thread-default-context aware.

Add a test program to verify that basic gio async ops work correctly
in non-default contexts.

http://bugzilla.gnome.org/show_bug.cgi?id=579984

gio/gfile.h

@@ -505,6 +505,7 @@ struct _GFileIface
   gboolean            (* stop_mountable_finish)       (GFile                *file,
                                                        GAsyncResult         *result,
                                                        GError              **error);
+  gboolean            supports_thread_contexts;
 };
 
 GType                   g_file_get_type                   (void) G_GNUC_CONST;
@@ -940,6 +941,8 @@ gboolean g_file_replace_contents_finish      (GFile                  *file,
 					      char                  **new_etag,
 					      GError                **error);
 
+gboolean g_file_supports_thread_contexts     (GFile                  *file);
+
 G_END_DECLS
 
 #endif /* __G_FILE_H__ */