Add a QPlatformPrinterSupportPlugin for Cocoa

Add a QPlatformPrinterSupportPlugin for Cocoa

This is used to create the actual QPrintEngine and to query the system
about printer information.

The QCocoaPrinterSupport actually lives in the Cocoa platform plugin,
so the QCocoaPrinterSupportPlugin uses the
createPlatformPrinterSupport() function in the QPlatformNativeInterface
(since the latter is a QObject). This is done to avoid adding a printing
related virtual function to QPlatformIntegration (since we have plans
to do a new printing API in the future).

Change-Id: I8bf9be668ff8ae1d07840e2af1e0846dc3299334
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>