QtPrintSupport: Move plugin access of QPrinterInfo internals to base

QtPrintSupport: Move plugin access of QPrinterInfo internals to base

Move all plugin methods requiring access to QPrinterInfo internals
into the plugin base class, and remove the plugin classes as friends
from QPrinterInfo.

Change-Id: Ic527efc681e198abf19e038dd77c36bb8017d049
Reviewed-by: Teemu Katajisto <teemu.katajisto@digia.com>
Reviewed-by: John Layt <jlayt@kde.org>