[INTERNAL] fix for commit ee8b5d69acf2413ecfa6f4e36859143f10517f7d

Due to the cleanup the logic that manages the editor input states was
changed in that way that it used always the returned results from an
IEditorPart. 

This is wrong as some values returned by those objects can change over
time.

This patch reintroducing the necessary logic.


Change-Id: Ie9b8fa5c8fbd907e664932dd8e4c722a35882658
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1897
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>