Add a way to prevent a dialog from closing

When the user presses the OK button on a QtQuick.Dialog having the default contentItem (with the proper standardButtons property set), the dialog is closed immediately, without giving the developer a chance to validate the contents.

One option could be adding a closing() signal like QtQuick.WIndow has, or add a parameter to the (undocumented) buttonClicked() signal, carrying an object with an "accepted" flag which can be set to true to prevent further processing of the event.