SI-6784 Localize feature imports in scala.swing.

These were inadvertantly added to the public API in f15dc74 for anyone
importing scala.swing._, which unintentionally enabled reflectiveCalls
and implicitConversions for clients of the API.

This change breaks the source compatibiliy poilicy, rather than
deprecating these, based on the extremely low chance that someone is
using these in something like:

    import scala.swing._; println(implicitConversions)
Address a deprecation warnings in scala-swing.

While I was in the neighbourhood.
Merge pull request #2465 from retronym/ticket/6784

SI-6784 Localize features imports in scala.swing