PlantUML view lines displays types instead of attributes

[abulka]

PlantUML view lines/edges displays types instead of attributes. Raised in #75.

The code responsible for generating PlantUML markup is def displaymodel_to_plantuml(displaymodel): in src/generate_code/gen_plantuml.py.

The 'display model' from which the PlantUML is derived, currently does not store the attribute information, so this is going to be a bit tricky to solve. The underlying pmodel (parse model) does have this information but once the UML is rendered, the pmodel is discarded - certainly not persisted. This needs careful thought.

[abulka]

Disabled line labels in PlantUML view for now, in 2df3c66, till this is resolved.