Add OstreeAsyncProgress, use it for ostree_repo_pull

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/.