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.

examples/gallery/pages_data_test.go

@@ -47,3 +47,13 @@ func TestDataDemoButtonsLog(t *testing.T) {
 		t.Fatalf("Light candles not logged: %q", *events)
 	}
 }
+
+func TestDataAccordionDemoLogsChange(t *testing.T) {
+	tt, events := dataDemo(t, "074")
+	if err := tt.Click("Bells"); err != nil {
+		t.Fatal(err)
+	}
+	if !slices.Contains(*events, "open [bells]") {
+		t.Fatalf("Accordion change not logged: %q", *events)
+	}
+}