QtPrintSupport - Modify Platform Plugin QPrinterInfo api

QtPrintSupport - Modify Platform Plugin QPrinterInfo api

Change the way the printsupport plugin creates QPrinterInfo
objects, provide platform api to return a named printer, and
expose this as static public api in QPrinterInfo.

Only the Mac plugin used the old api, the other plugins will
have direct support added in separate commits, but will use
the default implementation for now.

Change-Id: I7d6b6556eb39919cfb15bc0e814afbaf13c5712c
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>