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/072-tree.md

@@ -15,7 +15,7 @@ view := func(c *ui.Context) {
 		Roots:    []string{"Nave", "Sacristy"},
 		Children: func(n string) []string { return kids[n] },
 		Label:    func(n string) string { return n },
-	}).Size(280, 240)
+	}).Element.Size(280, 240)
 }
 ui.Render(view, 400, 300, 1)
 ```