message: matrix detail drag-drop
  ref surveyjs/survey-creator#2317
diff: @@ -137,6 +137,12 @@ export class DragDropSurveyElements extends DragDropCore<any> {
  if (dropTarget.getType() === "paneldynamic" && !this.isEdge) {
  dropTarget = (<any>dropTarget).template;
  }
  +
  + // drop to matrix detail panel
  + if ((dropTarget.getType() === "matrixdropdown" || dropTarget.getType() === "matrixdynamic") && (<any>dropTarget).detailPanelMode !=="none" && !this.isEdge) {
  + dropTarget = (<any>dropTarget).detailPanel;
  + }
  +
  // drop to panel
  else if (dropTarget.isPanel) {
  const panelDragInfo = this.getPanelDragInfo(
  