Fix disabling auto-suggestions (#13914) This should fix #13913 but I could not reproduce it in the first place. It looks like the traitlets initialization was invoking setter handler before the `__init__` had time to set `auto_suggest = None`; this is now changed so that `auto_suggest` is set to `None` on class level. It might be not as elegant because it makes it a class variable but in fact `pt_app` also uses the same pattern and since `auto_suggest` is only used on `pt_app` there is no reason not to use this pattern. Also, the type annotation was in a wrong class, I moved it.