Many enhancements and additions around <rbs-document-list/>, Collections and RichText editing.

* Added `<rbs-rich-text-input/>` to allow rich text edition in the Markdown format.
* Added options to `<rbs-document-list/>`: `collection-url` and `data-*` attributes, and value converters on columns (useful for Collection items rendering).
* Added `rbs-items-from-collection="<collection code>"` to feed a `<select/>` with the items of a named Collection.
* Editor has been optimized to only save the modified properties.