[Lazarus] Printers4Lazarus: QtDefaultPrinter

[Zoë Peterson via Lazarus]

I was just digging through the OSPrinters code and noticed that all of
the Qt code interacts with QtDefaultPrinter instead of
TQtPrinters.Handle (which is never assigned). Is that intentional?

Also, is printing from background threads (either just 1 or multiple
using separate TPrinter objects) supported on any of the widgetsets?