Feature Request: Notifications

A new feature that would allow for system-wide notifications to be sent to the headset via OpenXR. While developing multiplayer games that hinge on friend invites, you can run into a problem when developing the games for cross-platform. If you want to send a notification to the player while not playing the game, you have to use the system's respective api. This feature would make it a lot easier to create push notifications on more niche devices such as the hololens which have less documentation and support than OpenXR.

The issue names no files, tests, or entry points. Start by reviewing the OpenXR specification and its existing system-level interfaces, then determine the scope and expected behavior for notifications across devices. Done requires an agreed API design and corresponding documentation and validation plan.

Written by the indexing model from the issue text.