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/061-breadcrumb.md

@@ -33,7 +33,7 @@ if i, ok := mujica.Breadcrumb(c, path, mujica.BreadcrumbOptions{}).Chosen(); ok
 | `MaxItemWidth` | `float32` | 0（即 160 DIP） | 单项文字最大宽度，超出以省略号截断。 |
 | `Label` | `string` | 内置“路径导航” | 整组的无障碍名称。 |
 
-返回 `BreadcrumbResult`，内嵌 `*ui.Element`。
+返回 `BreadcrumbResult`，`Element` 字段是组件元素。
 
 ## 状态