Make it possible to easily expose vertex details:

 * Inspectable now has an empty default implementation of 'expose', a function
   which should return a Map[String,Any], and that will be serialized to JSON.
 * This way, any vertex can choose to expose any sort of information. The
   functionality is disabled by default because there can be a lot of data
   ivolved
 * SCC remembers which vertex has last been exposed and reloads the infos
   automatically when stepping etc.
 * The sidebar can be resized using the mouse
 * Plus a few cleanups all over