Comment 1 for bug 1207399

not a bug in ubuntu-ui-toolkit, this is already possible like this:

        Component {
            id: dialog
            Dialog {
                id: dialogue

                title: "Sample Dialog"
                text: "Are you sure you want to delete this file?"

                Button {
                    text: "Cancel"
                    gradient: UbuntuColors.greyGradient
                    onClicked: PopupUtils.close(dialogue)
                }
                Button {
                    text: "Delete"
                    onClicked: PopupUtils.close(dialogue)
                }
            }
        }