Rendering of marker values in margin-region

[bertfrees]

Note that in theory you can already accomplish this by creating a marker-indicator + class for each distinct value in the document.

To support this properly, I would suggest allowing marker-reference inside margin-region or indicators. The direction, scope and start-offset attributes would probably not be needed though, and if they would be needed they would have a different meaning. So maybe it's better to give the element a new name.

The behavior would be a mix of marker-reference and marker-indicator. It's like "marker-reference" because values of markers are inserted, not just symbols that indicate the presence of markers. And it's like "marker-indicator" because of where the strings are positioned: in left or right margin regions, and relative to a certain line.

By default the first marker on a line should be selected (this is also the only behavior I can think of atm that makes sense).