Add OstreeAsyncProgress, use it for ostree_repo_pull

Several APIs in libostree were moved there from the commandline code,
and have hardcoded g_print() for progress and notifications.  This
isn't useful for people who want to write PackageKit backends, custom
GUIs and the like.

From what I can tell, there isn't really a winning precedent in GLib
for progress notifications.

PackageKit has the model where the source has GObject properties that
change as async ops execute, which isn't bad...but I'd like something
a bit more general where say you can have multiple outstanding async
ops and sensibly track their state.

So, OstreeAsyncProgress is basically a threadsafe property bag with a
change notification signal.

Use this new API to move the GSConsole usage (i.e. g_print()) out from
libostree/ and into ostree/.

src/libostree/ostree.h

@@ -1,6 +1,6 @@
 /* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*-
  *
- * Copyright (C) 2011 Colin Walters <walters@verbum.org>.
+ * Copyright (C) 2011,2013 Colin Walters <walters@verbum.org>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -16,12 +16,11 @@
  * License along with this library; if not, write to the
  * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  * Boston, MA 02111-1307, USA.
- *
- * Author: Colin Walters <walters@verbum.org>
  */
 
 #pragma once
 
+#include <ostree-async-progress.h>
 #include <ostree-core.h>
 #include <ostree-repo.h>
 #include <ostree-mutable-tree.h>