QtQuick Dialogs: break out ButtonBox as a separate control

Users expect to have more control over the contents of a custom Dialog, so in retrospect it seems ButtonBox should be easier to add and remove from a Dialog. One way would be to make it optional, like the toolbar property can be set in ApplicationWindow; but that would cause existing applications to need rework.

Users are surprised that they can't do anchors.fill: parent on the content that they declare inside the dialog. So to accommodate that, maybe ButtonBox needs to be inside the contentItem, but there should be a sub-content Item which is a sibling, which is actually the default parent for items declared inside the Dialog. But that may still cause problems with auto-sizing: the dialog tries to resize its window to fit the contentItem's implicit height.