ItemsContentProvider: Keep the original image URIs in the database.

So we can use them (in a subsequent commit) to try to download them again,
if the previous download was interrupted somehow.
Restore some whitespace that I prefer.
ItemsContentProvider: Keep a list of image downloads in progress.
ItemsContentProvider: doRegularTasks: Download missing images.

This should deal with the situation where an image download fail, for instance when the network
connection is interrupted or there is a crash.