Add method to get NSPrintInfo* from QPrintEngine in Cocoa plugin

Add method to get NSPrintInfo* from QPrintEngine in Cocoa plugin

Add an invokable method that takes the QPrintEngine (which will
always be a QMacPrintEngine) and return the NSPrintInfo* from the
QMacPrintEnginePrivate. This will be used by the native dialogs in
QtPrintSupport to get/set printer settings.

Change-Id: If1e49027e8f0d505656db51be1f40a23f60e8e57
Reviewed-by: Mark Brand <mabrand@mabrand.nl>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>