QPrintDialog: document the modality on OS X and Windows

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

Task-number: QTBUG-32464
Change-Id: I2f24581dc660a088b4e741fa4ee1518e27adea4a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Windows: Mark window margins as dirty when the theme changes.

Task-number: QTBUG-31523

Change-Id: Ibd8ed4e3fc5b6d1723f94dc32b3bf86b74e71020
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Fix crashes when invoking toVariant() on empty QJsonValue objects.

Change-Id: I51cd114e862c6fad564484e990348f324ad56ab9
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>