Allow apps receiving a push import to set the ContentStore

[Robert Schroll]

Apps receiving a pull import (one they initiated) can set the ContentStore to have the content delivered to a permanent location. However, apps receiving a push import (initiated from without) cannot, and thus always receive content in the volatile ~/.cache directory. If they wish to save it permanently or be able to modify it, they must move it themselves.

Since apps will likely try to share code between the two import paths, I suspect this will cause most apps to ignore the ContentStore and handle file management manually in both cases. We should allow the ContentStore to be set on push imports as well, so that receivers can always make use of it.

[Ken VanDine (ken-vandine)]

On the receiving end, they should be able to change the ContentTranfers.store property after the transfer is charged, which would trigger a move on the hub side. This would effectively do the same thing the ContentItem.move() function does, but it would utilize the ContentStore API providing more consistency.