Add events to runtime

[jeffreyguenther]

Add events to the shiro runtime to allow graphical user interfaces to register handlers and respond to changes in the model

Some events that might be useful:

• port expression change
• node update
• graph update started
• graph update progress
• graph update complete

[jeffreyguenther]

I will take a different approach for events and will tackle this later when I make Shiro realtime.