Dialog containment should be reset when using new "appendTo" option

If a dialog is moved outside of its default position to an alternate element using the new "appendTo" option then the dialog's containment should be reset to that new element.

This is similar to #5699, except that now that there's an officially supported method of reparenting a dialog I think the user should expect jQueryUI to "do the right thing" automatically.

My generic code to reparent a dialog now looks like this:

            var $this = $(this);
            var o = $this.dialog('option');
            if (!o.modal) {
                $this.dialog('option', 'appendTo', $wm);

                var $p = $this.parent();
                if (o.draggable) {
                    $p.draggable('option', 'containment', 'parent')
                }

                if (o.resizable) {
                    $p.resizable('option', 'containment', 'parent');
                }
            }

which is IMHO needlessly complicated (and requires knowledge of jQueryUI internals that most developers don't need) when this logic should be inside the plugin.