To reliably test later HiDPI fixes, it is important to enable
HiDPI-rendering of pixmaps in test applications in the first place.
Tests without pixmaps have not been changed, the flag will be set by
default in Qt 6 anyway.
Test Plan:
`QT_SCALE_FACTOR=1.7 ./bin/kxmlguiwindowtest` does not show pixelated
icons anymore, except in places needing individual fixes (e.g. arrows in
"Configure Keyboard Shortcuts" dialog).
There are now three possible cases to handle: the bugs.kde.org wizard, a
custom email address, or a custom URL. The third case is handled in the
same way as bugs.kde.org, except that a web browser is simply opened on
the specified URL.
Differential Revision: https://phabricator.kde.org/D4072