dnd: Centralize drag actor positioning code, use shellWorkspaceLaunch for workspaces

We had multiple copies of the code to position a drag actor given a particular
source.  Instead, just put it inside dnd.js.

Second, rather than test for GenericDisplay/WellDisplayItem etc.,
in various places, add a new method on each source "shellWorkspaceLaunch"
which both marks the item as being droppable on a workspace, and is
called by the workspaces code to launch the item.

js/ui/docDisplay.js

@@ -83,9 +83,15 @@ DocDisplayItem.prototype = {
         }
     },
 
+    //// Drag and Drop ////
+
+    shellWorkspaceLaunch: function() {
+        this.launch();
+    },
+
     //// Private Methods ////
 
-    // Updates the last visited time displayed in the description text for the item. 
+    // Updates the last visited time displayed in the description text for the item.
     _resetTimeDisplay: function(currentSecs) {
         let lastSecs = this._docInfo.timestamp;
         let timeDelta = currentSecs - lastSecs;