[INTERNAL] cleaned up EditorPartListener

Made fields private and final, removed no needed null checks and
put the class Javadoc to the right place.

Change-Id: Icfd7f42a4fead23cedb2bb3b23260606703250d2
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1848
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>