Drag and drop fixes (#2249)

* Drag-drop edit mode from capture handler. Drag-drop composition from bubble handler. Check composability on drag start

* Show drop hints without being in edit mode.

* Don't serialize objects twice on drag

src/api/Editor.js

@@ -28,6 +28,11 @@ export default class Editor extends EventEmitter {
         super();
         this.editing = false;
         this.openmct = openmct;
+        document.addEventListener('drop', (event) => {
+            if (!this.isEditing()) {
+                this.edit();
+            }
+        }, {capture: true});
     }
 
     /**