Xpra: Ticket #2318: silence AT-SPI warnings

Looks like these:

** (Xpra:11592): WARNING **: 17:25:23.488: AT-SPI: Could not obtain desktop path or name
** (Xpra:11592): WARNING **: 17:25:23.491: atk-bridge: GetRegisteredEvents returned message with unknown signature
** (Xpra:11592): WARNING **: 17:25:23.492: atk-bridge: get_device_events_reply: unknown signature
** (Xpra:11592): WARNING **: 17:25:23.492: atk-bridge: get_device_events_reply: unknown signature

During the audio probing done using the hidden subcommand xpra _sound_query.

Strange thing is that this subcommand runs fine on its own. Something different in the environment perhaps? Or maybe we can blacklist the gstreamer plugin that causes this?

Exactly like this report: Odd AT-SPI Messages.

Ubuntu: Terminal and Nautilus stopped working after a crash points towards a dbus problem?

Gnome: Constant error messages "AT-SPI: Could not obtain desktop path or name" cause poor performance