Add discourse view based on annotations instead of tokens

[Amir Zeldes]

It should be possible to use the discourse view to view all annotations from a certain layer instead of the tokens. This is mainly a feature request from Falko right now, so you can see the whole document's Target Hypothesis (ZH1 etc.) as a coherent text. We should allow the user to configure the visualization in the resolver specifying the annotation layer to output (default is tok: the discourse view we have now). The visualizer should then output all annotations of that type for that document in plain text, ordered by the elements left token index and then by right token index (in principle it's possible to display nested annotations this way, though it would look terrible).