TCR fixes 2 (#2286)

* prevent default on dragover in dropHint, to allow drop event to fire

* add notebook snapshot to preview

* fix for preview image overlay

* pin fast-sass-loader version to 1.4.6

* fix saveAs in plot image export

* fix elements search in inspector

* fix current Search error

* fix anonymous render error in layout

* navigate and edit on create

* remove domainObjects from composition when deleting frames and containers, and also fix a bug whereby a user can add domainObject via drag and drop(composition) but because of the lack of containers, it will not be added to the flexible layout

* fix index undefined error when reordering containers

* throw an error when user cancels instead of returning false

* fixes for toolbar not updating on selection change

* fix errors when objects without context are returned by the search aggregator

* prompt user before cancelling edit

* check transactions before prompting user

* add save and continue editing option to save menu

* prompt user if in edit mode and is navigating away

src/api/composition/DefaultCompositionProvider.js

@@ -178,8 +178,12 @@ define([
      * @method remove
      */
     DefaultCompositionProvider.prototype.remove = function (domainObject, childId) {
-        // TODO: this needs to be synchronized via mutation.
-        throw new Error('Default Provider does not implement removal.');
+        let composition = domainObject.composition.filter(function (child) {
+            return !(childId.namespace === child.namespace &&
+                childId.key === child.key);
+        });
+
+        this.publicAPI.objects.mutate(domainObject, 'composition', composition);
     };
 
     /**