QQuickMessageDialog: Footer has a larger boundingBox than it should on non-basic styles

The new QQuickMessageDialog in QtQuick.Dialogs 2, currently has a larger footer than it should for non-basic styles. The footer's size is further increased once the user presses the "show details..." button, and by that point the footer will never be resized back to the old size value.

Steps to reproduce:
Run the qtdeclarative/tests/manual/quickdialogs/dialogs manual test project with the QT_QUICK_CONTROLS_STYLE environment variable set to either "Fusion", "Imagine", "Material" or "Universal", then open the MessageDialog.