RuntimeEnvironment should use def instead of val for abstract members

[danielkza]

The RuntimeEnvironment trait has all it's components as val. That means they cannot be overriden by a defs or lazy val, removing quite a bit of flexibility. For example, I wanted each of my controllers to register an event listener if desired, and do user-aware actions from some of them. But that is not possible because eventListeners is immutable. It also happens to be a List when it should be a Seq so it could be overridden by any type of collection.

[jaliss]

@danielkza just implemented this. thanks.