Review styles

[klauer]

Styles have been reworked to be slightly less cumbersome than the original C++ version, but it's not perfect:

• StyleCollection is a container for node, connection, and view styles
  • Can be loaded from JSON files, Python dictionaries, or just user-tweaked Style classes
• Styles are passed in as follows:
  • Scene takes in a collection; uses the connection and view styles
  • NodeDataModel takes in a collection; uses the node style