overview: Allow dropping icons on windows

Currently the user has to find an empty spot in the workspace
to be able to launcha new instance of an app using dnd.

This is unnecessary hard, so just allow dropping on windows too.

https://bugzilla.gnome.org/show_bug.cgi?id=652079

js/ui/workspace.js

@@ -308,6 +308,21 @@ WindowClone.prototype = {
         this.emit('drag-begin');
     },
 
+    _getWorkspaceActor : function() {
+        let index = this.metaWindow.get_workspace().index();
+        return Main.overview.workspaces.getWorkspaceByIndex(index);
+    },
+
+    handleDragOver : function(source, actor, x, y, time) {
+        let workspace = this._getWorkspaceActor();
+        return workspace.handleDragOver(source, actor, x, y, time);
+    },
+
+    acceptDrop : function(source, actor, x, y, time) {
+        let workspace = this._getWorkspaceActor();
+        workspace.acceptDrop(source, actor, x, y, time);
+    },
+
     _onDragCancelled : function (draggable, time) {
         this.emit('drag-cancelled');
     },

js/ui/workspacesView.js

@@ -149,6 +149,10 @@ WorkspacesView.prototype = {
         return this._workspaces[active];
     },
 
+    getWorkspaceByIndex: function(index) {
+        return this._workspaces[index];
+    },
+
     hide: function() {
         let activeWorkspaceIndex = global.screen.get_active_workspace_index();
         let activeWorkspace = this._workspaces[activeWorkspaceIndex];