Picto Xtext Example: add plantuml-based transformation and other views

[alfonsodelavega]

This PR adds a PlantUML-based transformation to visualise the entities of the Xtext's dmodel language:

• The domain model language now has a string doc element for each entity, which is shown as a note in the views
• Navigation between views and with the Xtext document has been maintained
• Layers support has been added (add/remove reference labels, supertypes and subtypes, and so on)

Additional views:

• A specific bar chart that counts datatypes, entities, features
• A pinset-based table that counts elements of each type, and the number of times they are used in the model (e.g. how many times a datatype is used in an entity feature)

The previous Graphviz transformation has been maintained as well, as it is referenced in the Xtext article. That article will be updated to reflect these new visualisations.