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/062-pagination.md

@@ -12,7 +12,7 @@ if mujica.Pagination(c, &page, &size, mujica.PaginationOptions{Total: 420, PageS
 }
 ```
 
-签名：`func Pagination(c *ui.Context, page, pageSize *int, opts PaginationOptions) PaginationResult`
+签名：`func Pagination(c *ui.Context, page, pageSize *int, opts PaginationOptions) ChoiceResult`
 
 ## 参数
 
@@ -28,7 +28,7 @@ if mujica.Pagination(c, &page, &size, mujica.PaginationOptions{Total: 420, PageS
 | `Siblings` | `int` | 0（即 1） | 当前页两侧显示的页码数；为负 panic。 |
 | `Disabled` | `bool` | false | 整体禁用。 |
 
-返回 `PaginationResult`，内嵌 `*ui.Element`。
+返回 `ChoiceResult`，`Element` 字段是组件元素。
 
 ## 状态