Add properties API to allow plugins to make suggestions to others

[timja]

See:

• Add Extension point for plugins to be able to contribute theming requirements (e.g. source code highlighting) #88
• Use recommended theme from theme manager prism-api-plugin#41
• Add prism configuration dark-theme-plugin#194

cc @uhafner what do you think of the API?

(I'll file PRs against dark theme, prism api and design-library as well) - complete

Also need to add an extension point to move the UI here but may be a separate PR

and will add docs once it's working end 2 end