workspaceThumbnails: Fix creating new workspace via DND

Commit 61a58ff3c981d4 replaced the (removed in commit 254afc50223a7)
MetaWindowActor.get_workspace() method by MetaWindow.get_workspace(),
but did not take into account that the return values differ - the
former returns the workspace index, the latter the workspace object.

https://bugzilla.gnome.org/show_bug.cgi?id=724686