rework APK downloading to mark files as partial or complete
Browsers have established a nice pattern for marking downloaded files as partial or complete: they add .part
to the file while its downloading, then only rename it back to the original name when the download is complete. The Updates screen needs to know the status of the files to accurately display the status of updates, especially if F-Droid was killed in the middle of the process.
see discussion in !547 (merged)