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/075-collapsible.md

@@ -19,7 +19,7 @@ ui.Render(view, 400, 300, 1)
 
 ## 参数
 
-`func Collapsible(c *ui.Context, open *bool, opts CollapsibleOptions, content func()) CollapsibleView`。
+`func Collapsible(c *ui.Context, open *bool, opts CollapsibleOptions, content func()) ChoiceResult`。
 
 | 字段 | 类型 | 默认值 | 含义 |
 | --- | --- | --- | --- |
@@ -33,7 +33,7 @@ ui.Render(view, 400, 300, 1)
 
 ## 事件
 
-`CollapsibleView.Changed()`：用户点击或按键切换了展开状态。
+`ChoiceResult.Changed()`：用户点击或按键切换了展开状态。
 
 ## 键盘操作