visualise overlapping denotations

[TimRepke]

At the moment one can annotate denotation boundaries without limitation. However, due to the structure of HTML, only nested denotations can actually be drawn, overlapping denotations cannot. To enable that, these cases have to be determined and instead of drawing one span for each denotation, three are needed (first part of first denotation, overlapping part, second art of second denotation).
Doing so will cause multiple problems upsteam, like how to deal with click events and drawing connections.