QtPrintSupport: Add CUPS printsupport plugin

QtPrintSupport: Add CUPS printsupport plugin

Move CUPS code around to create a new CUPS printsupport plugin, this
fixes QPrinterInfo for CUPS which depends on the plugin to work.

It QT_NO_CUPS is defined then the plugin is not built and only Print
to PDF is supported under Linux.

* Move unused genericiunixprintersupport plugin to start new CUPS
  printsupport plugin
* Split QPdfPrintEngine to create QCupsPrintEngine
* Remove LPR related code from QPdfPrintEngine
* Move CUPS specific code from plugin base class to derived CUPS class
* Remove forcing CUPS print engine to use PDF mode as PDF is now Native
* Move qt_getCupsPrinterPaperSizes from qprinterinfo_unix to
  QCUPSSupport
* Remove qprinterinfo_unix as no longer used
* Remove all QT_NO_LPR uses

There is now no CUPS specific code left in printsupport/kernel except
QCUPSSupport which is needed for the dialogs.

Task-number: QTBUG-23060

Change-Id: Ie8fa4512a2424edc8943068e0fa9fb714cc42db9
Reviewed-by: Teemu Katajisto <teemu.katajisto@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: John Layt <jlayt@kde.org>