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.

datetime.go

@@ -68,7 +68,7 @@ func (d Date) within(lo, hi Date) bool {
 
 // DateTimeResult is a date or time component and what happened to it this frame.
 type DateTimeResult struct {
-	*ui.Element
+	Element *ui.Element
 	// Input is the text field inside, for labels and focus; nil without one.
 	Input     *ui.Element
 	changed   bool