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/066-menubar.md

@@ -45,7 +45,7 @@ if id, ok := res.Chosen(); ok && id == "wrap" {
 | --- | --- | --- | --- |
 | `Label` | `string` | 内置“菜单栏” | 菜单栏的无障碍名称。 |
 
-返回 `MenubarResult`，内嵌菜单栏 `*ui.Element`。
+返回 `MenubarResult`，`Element` 字段是菜单栏元素。
 
 ## 状态