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.

command_palette.go

@@ -32,8 +32,8 @@ type CommandPaletteOptions struct {
 
 // CommandPaletteResult is the built CommandPalette; Element is nil while closed.
 type CommandPaletteResult struct {
-	*ui.Element
-	chosen string
+	Element *ui.Element
+	chosen  string
 }
 
 // Chosen returns the ID of the command run in this frame; running closes the palette.