Ability to add background images for each slide

[mdroidian]

https://revealjs.com/backgrounds/#image-backgrounds

from Ivo: https://roamresearch.slack.com/archives/C016N2B66JU/p1682172514925749

[kvistgaard]

The best would be if it could be in line with the current standard:

• setting: {layout:image background}
• rule: image in a first child block