Fix crash when changing tab titles preferences (#808)

- Switch `thunar_window_notebook_update_title` to be connected with `g_signal_connect_object`. This is necessary to keep the object alive during the function call.

- As well remove redundant signal disconnect. Since the signal for `thunar_window_notebook_update_title` was connected using `g_signal_connect_object`, it should be automatically disconnected
on destruction of the label.

MR !260