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-repo.h

@@ -24,6 +24,7 @@
 
 #include "ostree-core.h"
 #include "ostree-types.h"
+#include "ostree-async-progress.h"
 
 G_BEGIN_DECLS
 
@@ -461,6 +462,7 @@ gboolean ostree_repo_pull (OstreeRepo             *self,
                            const char             *remote_name,
                            char                  **refs_to_fetch,
                            OstreeRepoPullFlags     flags,
+                           OstreeAsyncProgress    *progress,
                            GCancellable           *cancellable,
                            GError                **error);