(libT) add package-visible API hook for when a block is downloaded. Add unit test to confirm that when the last file finishes downloading, its .part suffix is removed and it's moved from the incomplete to complete dir

git-svn-id: svn://svn.transmissionbt.com/Transmission/trunk@13879 f4695dd4-2c0a-0410-b89c-da849a56a58e