Refactor: results expose Element as a named field

Every component result type now names its element Element instead of
embedding *ui.Element. The eleven element-plus-changed results
(AccordionView, CarouselView, CollapsibleView, ImageViewerView,
PaginationResult, StepsResult, SidebarResult, TabsResult, RatingParts,
FileDropZoneParts, TransferParts) fold into ChoiceResult.

Fixes four Gallery demos where .Width(...).Changed() read the element's
flag instead of the component's: Accordion, MultiSelect, Cascader,
TreeSelect. TestDataAccordionDemoLogsChange covers the Accordion.

tree_table.go

@@ -122,7 +122,7 @@ func TreeTable[K comparable](c *ui.Context, s *TreeTableState[K], opts TreeTable
 	before := s.Sort
 	var v DataTableView
 	v.Element = ui.Column(c).MinHeight(0).Gap(d.Unit() * 2)
-	v.Children(func() {
+	v.Element.Children(func() {
 		if len(opts.Roots) == 0 {
 			t.clear(&s.Selection)
 			v.retry = dataPlaceholder(c, opts.Status, opts.Error, opts.Empty, msg(c, "list.empty"))