Overrides Java launch method to find existing Scala launches

Since Eclipes 3.8, a package private method is used by JavaLaunchShortcut
to find existing launch configurations for a given type.
Overrides the Java launch method to use different logic to find the Scala
launch configuraitons.

Fixes #1001447
(cherry picked from commit d68c6cff184125505e34f6d51b165bb3a24f5d32)
Merge pull request #510 from skyluc/backport/508

Backport/508