Creates an API for ScalaPlugin

ScalaPlugin has to be a concrete class for the Eclipse platform, so the implementation has been moved in the `internal` package.
In the public package, there is a trait with the API method of ScalaPlugin, and an object with a singleton, to follow the usual Eclipse design.
Most of the helper methods which were in the ScalaPlugin class and object have been moved to better location: helper classes, or related companion objects.
I would rename this one to either `apply()` (my favorite) or `instance`. I find it weird to say `ScalaPlugin.plugin` all the time :)