Avoid writing to preferences on helper creation

[mdickinson]

This PR fixes the PreferencesHelper to avoid writing to the preferences the first time a helper is created for a given Preferences object.

The issue was that on creation, PreferencesHelper._initialize was being called twice: once with notify=False and again with notify=True, as a result of the listener to the preferences trait. This PR avoids the second call by making that listener only run post creation (using post_init=True).

Closes #342
Closes #74

Checklist

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[mdickinson]

@flongford Would you be able to review?

[flongford]

LGTM. The use of an additional observe listener seems appropriate here and appears to resolve the reported issue.

I can't think of any immediate downstream consequences of this change, but I expect these can be simply handled in a case-by-case basis.