Maintain sort order when new articles are added

Backport of e95e0f8e

In Qt 4.7, QSortFilterProxyModels no longer maintain their sort order
unless explicitly told to.  This meant that new articles would always
end up at the bottom of the list being displayed, even if the list was
sorted newest-first.

Incidentally, I could have sworn I tried this fix already and it didn't
work.

CCBUG: 247473
FIXED-IN: 4.4.11