App Picker: Prevent drag if favorite-apps is locked.

App Picker: Prevent drag if favorite-apps is locked.

Because there's nothing (in single-monitor setups) that could
take the drop in this case.

* js/ui/appDisplay.js:
  AllView._loadApps(), FrequentView._loadApps(): Pass
  an isDraggable parameter when creating the AppIcons,
  depending on whether the favorite-apps key is locked.
  AppIcon._init(): Check for isDraggable in the params and
  do not create _draggable if it was specified, to prevent a
  drag from starting.
  AppIcon.popupMenu(): Check _draggable before trying to call
  fakeRelease on it.
* js/ui/dash.js: Dash._createAppItem(): Check AppIcon._draggable
  before trying to connect to its signals.

https://bugzilla.gnome.org/show_bug.cgi?id=741325