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/024-text-input.md

@@ -15,7 +15,7 @@ if r.Submitted() {
 
 签名：`func TextInput(c *ui.Context, value *string, opts TextInputOptions) InputResult`
 
-`InputResult` 内嵌外层 `*ui.Element`，`Input` 字段是内部编辑元素，`Changed()`、`Submitted()` 报告事件。
+`InputResult` 的 `Element` 字段是外层元素，`Input` 字段是内部编辑元素，`Changed()`、`Submitted()` 报告事件。
 
 ## 参数