We originally made it per-window in b502280c06 for
issue #228, but that was back when we still needed window IDs for stuff like
message.info.
Nowadays, there's no reason for it to be per-window anymore. The rest of the
download code can deal with one global download manager (because QtWebEngine has
one), and apart from QNAM code which wasn't used here anyways (as tab_id=None)
there was nothing using the window ID anymore.
Also see #3456 which was the original motivation for this change.
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