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/071-data-table.md

@@ -27,7 +27,7 @@ opts := mujica.DataTableOptions[relic, int]{
 }
 view := func(c *ui.Context) {
 	mujica.Use(c, mujica.Settings{})
-	mujica.DataTable(c, &state, rows, opts).Size(360, 240)
+	mujica.DataTable(c, &state, rows, opts).Element.Size(360, 240)
 }
 ui.Render(view, 400, 300, 1)
 ```