[Feature] Include some method to show custom notifications in game mode

[ananace]

Currently, there's no way for a background process to report a notification onto the UI when the deck is in game mode. I've found this to be a bit of a pain when running certain non-Steam games which perform long-lived tasks in the background, and also when doing some background jobs as user units.

If Steam would implement the org.freedesktop.Notifications interface in the game mode UI - or maybe add a steam:// URL for posting a simple notification into the feed, it would be very helpful as background tasks could then report status updates to the user.