appDisplay: Animate DnD app icons at the position they were dropped

Indicate whether dropping an app icon was successful or not by using the
newly added `animateLaunchAtPos()` API of AppIcon which starts a zoom
out animation of the icon at the position the drop happened.

To get the position of the drag actor, we have to forward the arguments
passed to `acceptDrop()` and `handleDragOver()` to the internal drag
handlers of the WorkspaceThumbnails. We can use this position directly
without transforming it to stage coordinates because the actor is a
child of `Main.uiGroup` and the animation actor will also be a child of
this container.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/121

js/ui/workspace.js

@@ -2032,6 +2032,9 @@ var Workspace = class {
             metaWindow.change_workspace_by_index(workspaceIndex, false);
             return true;
         } else if (source.app && source.app.can_open_new_window()) {
+            if (source.animateLaunchAtPos)
+                source.animateLaunchAtPos(actor.x, actor.y);
+
             source.app.open_new_window(workspaceIndex);
             return true;
         } else if (!source.app && source.shellWorkspaceLaunch) {