Document masonry

[PoignardAzur]

This was initially a supposed to be a small documentation pass, which grew into a few changes:

• Adding back a to-do-list example.
• Fixing the bugs revealed by that example (infinite bounding boxes, wrong accessibility handling in Portal, etc).
• Making sure all widgets return the correct spans instead of the less-useful default one.
• Adding a trace to the layout pass for easier debugging.

Overall I'm pretty happy with this!