Fix traitlet _notify_trait by-ref issue

This fixes a problem with the existing traitlet machinery, where `anytrait` values get added to the notifiers list every time the `_notify_trait` function is called.

The bug is that each time the traitlet changes, the `anytrait` callbacks will each be called N times, where N is the number of times the traitlet has been changed (in the current application instance).