New event system

[ylt]

Current approach is a bit of a hack, especially in terms of how it's a nested object with the this scope of parent.

Should use a more typical approach, probably implement a plugininst.on("event_name", func). This would have the benefit of being able to optimise events and would make it easier to implement prioritisation (issue #20).

[ylt]

Progress towards this has been made using decorations (part of #56).

This still needs implementing, but the decorations can be modified to use this approach internally.