PreferencesHelper unable to synchronize mutations on nested containers

[kitchoi]

Similar to #129, currently mutations on nested containers for traits defined on a subclass of PreferencesHelper cannot be synchronized with the Preferences.

e.g.

from apptools.preferences.api import Preferences, PreferencesHelper
from traits.api import Int, List, Str


class MyPreferencesHelper(PreferencesHelper):
    preferences_path = Str('my_section')
    list_of_list_of_str = List(List(Str))

pref = Preferences(filename="tmp.ini")
helper = MyPreferencesHelper(preferences=pref, list_of_list_of_str=[["1"]])
helper.list_of_list_of_str[0].append("9")
pref.save()

The saved preferences would be:

[my_section]
list_of_list_of_str = [['1']]

Expected:

[my_section]
list_of_list_of_str = [['1', '9']]

[kitchoi]

Note that the example above with nested List of List did happen to work prior to Traits 6.1. More specifically, before this fix: enthought/traits#1018
But if it was a list of dict or a list of set, it still wouldn't work.