'Creates and the workspace model. Also tries to assign
self.default_selection to the tree_iter which represents the
last active workspace'

def create_ws_model(self):

    workspace_model = Gtk.ListStore(str)
    self.default_selection = None
    self.valid_ws_iters = []
    for ws in self.workspaces:
        tree_iter = workspace_model.append([ws])
        self.valid_ws_iters.append(tree_iter)
        if (ws == self.last_workspace):
            self.default_selection = tree_iter
    return workspace_model
