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.

button_group.go

@@ -28,7 +28,7 @@ type ButtonGroupOptions struct {
 
 // ButtonGroupResult is the group element and the item clicked this frame.
 type ButtonGroupResult struct {
-	*ui.Element
+	Element *ui.Element
 	clicked int
 }