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/050-transfer.md

@@ -34,7 +34,7 @@ ui.Render(view, 400, 300, 1)
 
 ## 事件
 
-`TransferParts.Changed()` 有条目移动。
+`ChoiceResult.Changed()` 有条目移动。
 
 ## 键盘操作