QQuickMessageDialog: Remove binding loop in non-native implementation

Currently, when creating a non-native (QtQuick.Dialogs 2) MessageDialog, the qml engine will warn about binding loops. Meaning that there has to be something not quite right about the current implementation.

qrc:/qt-project.org/imports/QtQuick/Dialogs/quickimpl/qml/MessageDialog.qml:45:1: QML MessageDialog: Binding loop detected for property "implicitWidth"
qrc:/qt-project.org/imports/QtQuick/Dialogs/quickimpl/qml/MessageDialog.qml:45:1: QML MessageDialog: Binding loop detected for property "implicitWidth"
qrc:/qt-project.org/imports/QtQuick/Dialogs/quickimpl/qml/MessageDialog.qml:45:1: QML MessageDialog: Binding loop detected for property "implicitHeight"
qrc:/qt-project.org/imports/QtQuick/Dialogs/quickimpl/qml/MessageDialog.qml:45:1: QML MessageDialog: Binding loop detected for property "implicitHeight"
qrc:/qt-project.org/imports/QtQuick/Dialogs/quickimpl/qml/MessageDialog.qml:45:1: QML MessageDialog: Binding loop detected for property "implicitWidth"

steps to reproduce:
run tst_QQuickMessageDialogImpl::changeStandardButtons(), tst_QQuickMessageDialogImpl::detailedText()
or the manual test project in qtdeclarative/tests/manual/quickdialogs/dialogs