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.

docs/components/073-tree-table.md

@@ -22,7 +22,7 @@ view := func(c *ui.Context) {
 			{ID: "size", Title: "Size", Width: 80, Align: ui.End,
 				Cell: func(n string) { ui.Text(c, strconv.Itoa(size[n])) }},
 		},
-	}).Size(360, 240)
+	}).Element.Size(360, 240)
 }
 ui.Render(view, 400, 300, 1)
 ```