Add hovering to block diagram viewer; rendering refactor

[ducky64]

Changes to block diagram rendering:

• array ports rendered as one port (instead of one for each element)
• fix tunnel labels not showing up in PDF
• highlight hovered element on interactive block diagram viewer

Internal refactoring:

• Instead of a ModifiedElkNodePainter that is aware of concepts like selection and stale, the base ElkNodePainter takes in per-element graphics modifiers that provides for fill, stroke, text, and (optionally) outline
  • Multiple modifiers will compose, each acts as a Graphics2D -> Graphics2D transformer
  • The color of a block's fill is passed in as the background color to inner blocks' fill