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/.
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