Refactor: EditableTextView.Changed() and the Canceled spelling

EditableTextView.Changed is a method like every other result's.
AlertDialogResult, PopconfirmResult and DateTimeResult spell Canceled
the American way, matching EditableTextView and LoadingOverlayResult.
Callers, Gallery and docs updated.

date_range_picker_test.go

@@ -17,7 +17,7 @@ func dateRangeTester(value *DateRange, opts DateRangePickerOptions, changes, can
 			if r.Changed() {
 				*changes++
 			}
-			if r.Cancelled() {
+			if r.Canceled() {
 				*cancels++
 			}
 		})