Add support for more common `Node` `Socket` layout patterns

[mitchmindtree]

This is a follow-up to #32.

The goal is to provide common high-level node+socket layouts that users are likely to want.

For example:

• Tabular horizontal layout:
  • Nodes are laid out as tables.
  • Their input/output socket locations can be inferred from the associated widget row position.
  • Similar to touch designer, grasshopper, unity.
• Vertical compact layout:
  • Current vertical behaviour.
  • Sockets are evenly spaced.
  • Requires hovering over a socket to know what it maps to.
  • Similar to max/msp, puredata.