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.

carousel_test.go

@@ -20,7 +20,7 @@ func (h *carouselHarness) view(c *ui.Context) {
 		v := Carousel(c, &h.index, 4, h.opts, func(i int) {
 			ui.Text(c, fmt.Sprintf("Window %d", i+1)).Padding(20)
 		})
-		v.Grow(1)
+		v.Element.Grow(1)
 		if v.Changed() {
 			h.changes++
 		}
@@ -126,7 +126,7 @@ func TestCarouselSnapshot(t *testing.T) {
 				ui.Column(c).Grow(1).Center().Children(func() {
 					ui.Text(c, fmt.Sprintf("Rose window %d", n+1)).FontSize(FontSize(c, 18))
 				})
-			}).Grow(1)
+			}).Element.Grow(1)
 		})
 	})
 }
@@ -135,7 +135,7 @@ func TestCarouselEmptyAutoplayKeys(t *testing.T) {
 	index := 0
 	tt := ui.NewTester(func(c *ui.Context) {
 		Use(c, Settings{})
-		Carousel(c, &index, 0, CarouselOptions{Autoplay: true}, func(int) {}).Size(300, 200)
+		Carousel(c, &index, 0, CarouselOptions{Autoplay: true}, func(int) {}).Element.Size(300, 200)
 	}, 320, 240)
 	for range 4 {
 		tt.Key(0, ui.KeyTab)