WorkspaceSwitcher: simplify code for handling keybindings

Most of code implementing workspace switches was repeated with
minor differences on each direction. Instead, consolidate it
and use the new meta_workspace_get_neighbor.

https://bugzilla.gnome.org/show_bug.cgi?id=674104

js/ui/workspacesView.js

@@ -1048,10 +1048,10 @@ const WorkspacesDisplay = new Lang.Class({
     _onScrollEvent: function (actor, event) {
         switch ( event.get_scroll_direction() ) {
         case Clutter.ScrollDirection.UP:
-            Main.wm.actionMoveWorkspaceUp();
+            Main.wm.actionMoveWorkspace(Meta.MotionDirection.UP);
             break;
         case Clutter.ScrollDirection.DOWN:
-            Main.wm.actionMoveWorkspaceDown();
+            Main.wm.actionMoveWorkspace(Meta.MotionDirection.DOWN);
             break;
         }
     }