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.

tabs_test.go

@@ -87,7 +87,7 @@ func TestTabsOverflowScrolls(t *testing.T) {
 	tt := ui.NewTester(func(c *ui.Context) {
 		Use(c, Settings{})
 		ui.Column(c).Width(240).Children(func() {
-			w = Tabs(c, &sel, tabs, TabsOptions{}).Bounds().W
+			w = Tabs(c, &sel, tabs, TabsOptions{}).Element.Bounds().W
 		})
 	}, 400, 200)
 	tt.Key(0, ui.KeyTab)