BUG: Instantiate DICOM exportables with a parent to avoid deconstruction

The crash was in DICOM export, at the following line:
https://github.com/Slicer/Slicer/blob/master/Modules/Scripted/DICOMLib/Widgets/qSlicerDICOMExportDialog.cxx#L263
Also if there was no crash (sometimes it happened), the exportables were the same in the list (i.e. the same series is displayed 3 times instead of showing the 3 series in the study). The issue was most probably that the exportable objects get deconstructed during passing from python to C++. I downloaded a nightly just before this commit
https://github.com/Slicer/Slicer/commit/9b7171d097ad0cfa1fe89780a2ef0758cf2ae80e
and indeed, the crash was not there, the exportables showed up correctly. This commit fixes the issue, but may not be the best solution overall.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24276 3bd1e089-480b-0410-8dfb-8563597acbee