Respect the custom paper size settings on Mac

When the user added a custom paper size then it would be silently
ignored when printing on Mac. This now ensures that it is respected when
appropriate.

[ChangeLog][Platform Specific Changes][OS X][QtPrintSupport] Respect
the custom paper size settings when printing.

Task-number: QTBUG-34700
Change-Id: I08afe24e0e67a50e9301abf4642c6f65bb0df1fe
Reviewed-by: John Layt <jlayt@kde.org>