When a download is finished with `removed-finished-download` set to a
delay, it's removed via a singleshot QTimer.
However, when the window was closed in the meantime, the slot still was
executed by Qt, even though the DownloadManager was already deleted.
Fixes#1242
Issue #1214
Now uses a sensible filename for data: links instead of the whole base64
content. For PDF.js, it even uses the correct pdf filename.
TODO: Produces "QPainter:🔚 Painter ended with 2 saved states" while
running the tests here (Arch Linux):
CPython: 3.5.1
Qt: 5.5.1, runtime: 5.5.1
PyQt: 5.5.1
