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/001-app-shell.md

@@ -29,7 +29,7 @@ mujica.AppShell(c, &collapsed, mujica.AppShellOptions{
 | `SidebarWidth` | `float32` | 240 | 展开时侧栏宽度（DIP）；负数 panic。 |
 | `ContentScroll` | `*ui.ScrollState` | nil | 保存主内容滚动位置；nil 时按位置保留。 |
 
-返回 `AppShellResult`，内嵌 `*ui.Element`。
+返回 `AppShellResult`，`Element` 字段是组件元素。
 
 ## 状态