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/063-steps.md

@@ -14,7 +14,7 @@ mujica.Steps(c, &current, []mujica.Step{
 }, mujica.StepsOptions{Label: "Sign-up"})
 ```
 
-签名：`func Steps(c *ui.Context, current *int, steps []Step, opts StepsOptions) StepsResult`
+签名：`func Steps(c *ui.Context, current *int, steps []Step, opts StepsOptions) ChoiceResult`
 
 ## 参数
 
@@ -36,7 +36,7 @@ mujica.Steps(c, &current, []mujica.Step{
 | `Vertical` | `bool` | false | 纵向排列，连接线在标记下方。 |
 | `Label` | `string` | "" | 整组的无障碍名称。 |
 
-返回 `StepsResult`，内嵌 `*ui.Element`。
+返回 `ChoiceResult`，`Element` 字段是组件元素。
 
 ## 状态