BUG: Fixed DICOM export crash

Nodes added to subject hierarchy now have a default level of 'Series' unless explicitly specified (if level of a data node is not Series then it cannot be exported to DICOM); Fixed crash when auto-selecting node in the DICOM export window subject hierarchy tree, due to a mismatch in model of given selection index and the model of the tree. Fixed by removing the selection code, as selection is made automatically by setCurrentNode call (it was not like this before; for the record, fixing the mismatch crash is possible by calling mapFromSource function of the sort filter proxy model with the main model index as argument, and setting selection with the sort filter model index)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24224 3bd1e089-480b-0410-8dfb-8563597acbee