Dialog: for back-compat, make titleBar optional.
Fixes #18253, and dojox/image/Lightbox.
Fix resizing of Dialog with dijitDialogPaneActionBar.

When the dijitDialogPaneActionBar is a sibling of this.containerNode, like in ConfirmDialog,
then resizing the Dialog smaller than its content will keep the OK/Cancel buttons visible,
and add the scrollbar just to this.containerNode.

Fixes #18316.