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.c

@@ -1454,6 +1454,7 @@ ostree_repo_read_commit (OstreeRepo   *self,
  * @remote_name: Name of remote
  * @refs_to_fetch: (array zero-terminated=1) (element-type utf8) (allow-none): Optional list of refs; if %NULL, fetch all configured refs
  * @flags: Options controlling fetch behavior
+ * @progress: (allow-none): Progress
  * @cancellable: Cancellable
  * @error: Error
  *
@@ -1467,6 +1468,7 @@ ostree_repo_pull (OstreeRepo               *self,
                   const char               *remote_name,
                   char                    **refs_to_fetch,
                   OstreeRepoPullFlags       flags,
+                  OstreeAsyncProgress      *progress,
                   GCancellable             *cancellable,
                   GError                  **error)
 {