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.
It is invoked through the IStatusHandler of Eclipse framework. Mainly, it allows to display dialog to the user from a not-UI plugin. In this case, when inconsistency is detected in a project classpath, like libraries cross-compiled on different versions.
@huitseeker knows more of the internal details, and should be the one adding the documentation.