filebrowser: added drag and drop for symbol view

vispa/extensions/file/static/html/symbol.html

@@ -8,7 +8,7 @@
 </div> -->
 
 <div class="file-view-symbol file-view-content-container">
-    <div class="file-node symbol-item" data-bind="data">
+    <div draggable="true" class="file-node symbol-item" data-bind="data">
         <p class="file-selection-p">
             <label class="file-selection-box">
                 <input type="checkbox" class="file-selection-checkbox">