enhance ctk dicom browser export series

[achataigner]

ctkDICOMBrowser::exportSeries export the series as DICOM files in a target directory.

I'm working on an application which listens to the target directory and tries to create volumes in real time by loading on the fly the DICOM files written by ctkDICOMBrowser::exportSeries.

ctkDICOMBrowser::exportSeries copies the DICOM files in the target directory:

• in an arbitrary order.
• by calling QFile::copy which creates temporary files

It would be great to write the files in the alphabetical order and without creating any temporary file.

Here is the pull request: https://github.com/commontk/CTK/pull/974