Bug 1043293 - Followup: don't launch an app if a contextmenu has been handled

* remove stopImmediatePropagation from some locations so that the contextmenu
  event can be received by the grid
* if the event is defaultPrevented, we don't do the default action, that is
  neither drag&drop (edit mode) or launch an app
Bug 1043293 - Follow up: fix integration tests