libostree: Add ostree_async_progress_copy_state()

This allows copying the state from one OstreeAsyncProgress object to
another, atomically, without invoking the callback. This is needed in
libflatpak, in order to chain OstreeAsyncProgress objects so that you
can still receive progress updates when iterating a different
GMainContext than the one that the OstreeAsyncProgress object was
created under.

See https://github.com/flatpak/flatpak/pull/3211 for the application of
this API.

apidoc/ostree-sections.txt

@@ -3,6 +3,7 @@
 OstreeAsyncProgress
 ostree_async_progress_new
 ostree_async_progress_new_and_connect
+ostree_async_progress_copy_state
 ostree_async_progress_get_status
 ostree_async_progress_get
 ostree_async_progress_get_variant