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/plugins/flexibleLayout/plugin.js

@@ -44,6 +44,7 @@ define([
                         containers: [new Container.default(50), new Container.default(50)],
                         rowsLayout: false
                     };
+                    domainObject.composition = [];
                 }
             });