QtPrintSupport - Cleanup QPrintDialog header

QtPrintSupport - Cleanup QPrintDialog header

Move QUnixPrintWidget from the header to the UNIX implementation as
it is not used anywhere else and doesn't need to be exported.

Make the private slots only compile under UNIX, remove the Mac and Win
stubs for them as unneeded.

Add some documentation to the UNIX dialog

Change-Id: I8f30504285df877e17ac8eeace875f666f43d94d
Reviewed-by: Teemu Katajisto <teemu.katajisto@digia.com>
Reviewed-by: John Layt <jlayt@kde.org>