Fix a loop in ItemRetriever and make it interactive, add a unit test

Fix a loop in ItemRetriever when we have more parts available
than requested as well as when only some requested parts need
to be retrieved.

Make ItemRetriever interactive by emitting a signal with list
of already retrieved items and using QEventLoop to block inside
ItemRetriever::exec() instead of blocking on a QWaitCondition
inside ItemRetrievalManager.

Add a unit-test with several different scenarios.