Handling connections better in DICOM export

[cpinter]

Current possibility: one plugin offers all the objects to export, and maybe calls other plugins to do the actual export Problem: If we want to export a more complicated study, we cannot do that in one export session Possible fix: The plugins tell the central plugin handler their required connections. It collects them, and exposes some more advanced view to the user when selecting the plugins (step 2. of the export workflow, under the tree)

Migrated from https://app.assembla.com/spaces/slicerrt/tickets/686-handling-connections-better-in-dicom-export/details

[cpinter]

Plan: http://www.na-mic.org/Wiki/index.php/2015_Winter_Project_Week:DICOM_References