G_APPLICATION_FLAGS_NONE was deprecated, but we don't actually need the single
instance functionality, so just use a non-deprecated flag instead.
This fixes Debian CI.
Note that g_assert() should not be used in unit tests, since it is a
no-op when compiling with G_DISABLE_ASSERT. Use g_assert() in production
code, and g_assert_true() in unit tests.
See https://github.com/fwupd/fwupd/issues/3790
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.
