[NiklasRentzCAU]

Instead of the previous "API", where users simply overwrote the generated code, which SPViz automatically reset on code regeneration, this PR adds a concrete API for changing the style of visualizations.

• Many more methods and fixed values in the Styles.xtend file are converted to public methods.
• Styles.xtend should now be extended with manual customizations in that extension.
• The extension should be located in the src/ folder, so it is exempt from automatic overwriting.
• The styles file can be registered via a service in the META-INF/services folder.
• This service is then passed automatically to the synthesis via Service Loader and Guice.

This makes it easy to add new behavior to the existing synthesis with a new class, and to check that class into Git without disturbing the rest of the generation.