QtPrintSupport: Change QPrinterInfo::defaultPrinter() behaviour

QtPrintSupport: Change QPrinterInfo::defaultPrinter() behaviour

In Qt4 QPrinterInfo::defaultPrinter() has inconsistant behaviour
depending on the platform.  On Mac and Unix if no default printer
is set in the system then the first available printer is returned.
but on Windows it returns a null QPrinterInfo.

Currently Qt5 returns the first printer on all platforms, but this
causes an inconsistency if you call isDefault() on the returned
QPrinterInfo.

The apidox is slightly ambiguous, but does seem to imply no printer
will be returned if no default is set.

The only place in Qt the public api is used is in QPrinter which is
coded expecting that defaultPrinter() may return null but for there
still to be availablePrinters().

Change-Id: I1bbef8cba259b7d51980a0199e9fd6d5466d921f
Reviewed-by: Teemu Katajisto <teemu.katajisto@digia.com>
Reviewed-by: John Layt <jlayt@kde.org>