wlr_scene: Add output_add event to wlr_scene.

Adds signal to wlr_scene that will notify listeners when a new output is added.

This enables managing frame scheduling entirely separately from other concerns. A scheduler can listen for outputs to be added to its scene, subscribe to frame events on those outputs, apply whatever magical timing logic, and then call wlr_scene_buffer_send_frame_done when appropriate. No input is required other than the scene to manage.

Tested here https://github.com/bwhmather/hayward/blob/master/hayward/src/scheduler.c.