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.

examples/gallery/pages_layout.go

@@ -417,7 +417,7 @@ func layoutSplitPaneDemo(c *ui.Context, log func(string)) {
 				Collapsible: sides[s.collapsible]},
 				pane("Files", "Drag the divider, or focus it and use the arrows.", k.Surface),
 				pane("Editor", "Enter on the divider folds the collapsible pane.", k.Background))
-			r.Fill()
+			r.Element.Fill()
 			if r.Resized {
 				log(fmt.Sprintf("resized to %.0f", s.split.Size))
 			}