[FIX] shared file decoration failed for non shared editors

The labels were not properly updates when a user switched to a non
shared editor.

Additional to this fix the class is now THREAD-SAFE !

Change-Id: I9a2de0e23a5ed4c2ef22641b9f183db9a3dfde4c
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1122
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>