Fixed drag-n-drop reorder of gallery images not responding to drops

no issue
- #1081 introduced a bug where the `insertIndex` was being cleared every time we showed the drop position indicator because we hide the indicator as a reset every time we show the indicator. This meant every drop was missing the required insertIndex.
- added a flag to the `_hideDropIndicator` function so that a reset when showing the indicator doesn't clear the required information

lib/koenig-editor/addon/services/koenig-drag-drop-handler.js

@@ -341,8 +341,8 @@ export default Service.extend({
     _showDropIndicator({direction, position, beforeElems, afterElems}) {
         let dropIndicator = this._dropIndicator;
 
-        // reset everything before re-displaying indicator
-        this._hideDropIndicator();
+        // reset everything except insertIndex before re-displaying indicator
+        this._hideDropIndicator({clearInsertIndex: false});
 
         if (direction === 'horizontal') {
             beforeElems.forEach((elem) => {
@@ -405,12 +405,15 @@ export default Service.extend({
         // TODO: handle vertical drag/drop
     },
 
-    _hideDropIndicator() {
+    _hideDropIndicator({clearInsertIndex = true} = {}) {
         // make sure the indicator isn't shown due to a running timeout
         run.cancel(this._dropIndicatorTimeout);
 
-        // clear droppable insert index
-        delete this.draggableInfo.insertIndex;
+        // clear droppable insert index unless instructed not to (eg, when
+        // resetting the display before re-positioning the indicator)
+        if (clearInsertIndex) {
+            delete this.draggableInfo.insertIndex;
+        }
 
         // reset all transforms
         this._transformedDroppables.forEach((elem) => {