QPrintDialog: document the modality on OS X and Windows

You can't programmatically close the dialog because it's modal.

Task-number: QTBUG-32464

(backported from qtbase/bd602a2dc4bc9491f80919b09a8990edb6fbee97)

Change-Id: Ib7573b347b3f10914e60af05f95ed9f5763eadd8
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
QPrintDialog OSX: don't crash if ApplicationModal and no parent

Task-number: QTBUG-32464

(backported from  qtbase/03855decfcb4e05817424843564a4b66636bb347)

Change-Id: I12892798419b2efcf404f972930562d50d045ea3
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
printsupport: set duplex setting correctly to CUPS printer

If CUPS printer default is not DuplexNone then setting duplex
to None for printer did not have any effect on duplex setting.

(backported from qtbase/385e584926f2957d6e9ec4c2c4e89b37a5e0c80c)

Change-Id: Ia72f599a9e5fa511b3214c6b8016e21f40409b86
Reviewed-by: Andy Shaw <andy.shaw@digia.com>