(io): execute PACS queries from a worker

Description

::ioPacs::SeriesPuller now retrieves series in a worker to avoid the app to freeze.

Closes #599 (closed)

How to test it?

• Launch OgreViewer.
• Click on the second button : Open PACS.
• Now you can retrieve DicomSeries from the PACS, preview them, and pull the it to an ImageSeries.

Check if the icon used to pull the series is disabled during the download process and if the app is not freeze.