Fix QuickOpen with Qt 5.11

It remains to be investigated whether this is a regression in Qt
itself, or misuse of the API on our side. There is at least one
misuse (the call to mapFromSource) which is wrong in the old version,
so this patch improves the situation in either case.

Differential Revision: https://phabricator.kde.org/D13517
BUG:395039