Updated core such that rotation center only is calculated after a file has been loaded. Especially not when switching from some picking mode back to examiner mode. This leads to beeing forced to calculate rotation center after having moved an object via the move manipulator since its rotation center stays on the previous position unless its not updated explicitly.

git-svn-id: http://www.openflipper.org/svnrepo/OpenFlipper/branches/Free@6139 383ad7c9-94d9-4d36-a494-682f7c89f535