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.

example_layout_test.go

@@ -100,7 +100,7 @@ func ExampleSplitPane() {
 		mujica.SplitPane(c, &split, mujica.SplitPaneOptions{MinFirst: 120, Collapsible: mujica.SplitPaneFirst},
 			func() { ui.Text(c, "Files") },
 			func() { ui.Text(c, "Editor") },
-		).Fill()
+		).Element.Fill()
 	}
 	ui.Render(view, 400, 300, 1)
 }