Use multi-selection for DICOM export

[cpinter]

It is now possible to select multiple nodes in subject hierarchy, and it would be good if the user could use that feature to select objects to export to DICOM.

• On multi-selection, all context menu options are disabled except for Delete. It will be needed to implement handling of "multi-enabled" actions
• The selected series could be in different studies or could be outside any study. How to auto-fill the patient/study fields?

Migrated from https://app.assembla.com/spaces/slicerrt/tickets/768-use-multi-selection-for-dicom-export/details