Disable useless and expensive Java goodies for Scala sources.

* quick fixes and quick assists in Scala sources have no meaning, but may trigger pauses in the UI of more than 10 seconds
* Run As/Debug As testers may trigger hierarchy calculations, inducing 10s of seconds of delay in the UI thread, for instance when scrolling through a contextual menu. This disables 'Run As Java'

Fixed #1001434. Fixed #1001178.
Merge pull request #278 from dragos/issue/disable-quick-java-assists-1001434

Disable useless and expensive Java goodies for Scala sources.