First steps towards a dialogue system

VERY MUCH A WORK IN PROGRESS :)
This likely wont compile to web/native game without uncommenting the
OnDrawGizmos method and the 'using UnityEditor' lines in DialogueNode.cs

Everything other than the basic dialogue trees is missing. No proper UI
for selecting dialogue options yet. Try talking to AlterAbner, see
options in the log and choose with 1-3 keys