Bug 1043293 - [Verticalhome] use "touchend" instead of "click" to launch applications r=kgrandon

* attach the scroll event on the main window
* 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