Compute positioning of single cues

This implements the part of the processing algorithm that positions single cues. I'd like to get some feedback on this before I proceed further.

What's left to do after this is to implement the part of the algorithm that adjusts the positions of cues in relation to the other cues that are being displayed and to implement the part that handles regions as well.

We also have to figure out the bidirectional paragraph problem and add more tests to make sure that we are hitting every path in the processing model algorithm. I think some students might be able to help us with that.

Partial fix for #89 and #86.