We don't want the (shared) progress notifications going to the thread-default
GMainContext by default as this changes behaviour (ABI?) with older libfwupd
versions.
If the client really does want progress notifications to go to a different
GMainContext they it can call fwupd_client_set_main_context() with the desired
context, e.g. g_main_context_get_thread_default() or g_main_context_new().
This has the intended side effect of returning progress and state notifications
to the main thread, which is typically (but not always) the thread that created
the FwupdClient object.
