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/055-time-range-picker.md

@@ -20,7 +20,7 @@ ui.Render(view, 400, 300, 1)
 
 ## 参数
 
-`func TimeRangePicker(c *ui.Context, value *TimeRange, opts TimeRangePickerOptions) DateTimeResult`，返回值嵌入 `*ui.Element`。
+`func TimeRangePicker(c *ui.Context, value *TimeRange, opts TimeRangePickerOptions) DateTimeResult`，返回值的 `Element` 字段是组件元素。
 
 | 字段 | 类型 | 默认值 | 含义 |
 | --- | --- | --- | --- |