Adds configuration of the project during import

The project is created as a Java project, then the classpath is configured according to the
data extracted from sbt, and finally the Scala nature is added.
Tweaks the import wizard UI

Disabled the working set section, has it is non-functionning right now.
Added a disabled section about which builder to use, to show that it is using the internal one.
Removed an extra border on the first label.