Backport improvements from petrinet renderer at petrinets.org

[epost]

The improvement that's being backported is drawing nets within bounds. They used to get drawn where the net wasn't centred and parts wouldn't be visible.

• Take changes from https://github.com/PetriNets/petrinets.github.io/blob/master/js/app.js as needed.
• Plz get rid of the backtick string interpolation syntax in favour of good old "..." + "..." to ease FFI integration.