Improve code quality of visualization-related components

[djnemeth]

The following modules have to be revised:

• hu.elte.txtuml.export.javascript
• hu.elte.txtuml.export.papyrus
• hu.elte.txtuml.layout.export
• hu.elte.txtuml.layout.visualizer

Possible improvements:

• eliminate code duplication
• remove unnecessary dependencies
• find better abstractions
• increase separation

This is to make future developments (e.g. #576, #534 and possibly #214) easier.

[djnemeth]

I recommend to address this in branch visualization-refactoring.