Fix traitlet _notify_trait by-ref issue
Added test for traitlet notify by-ref fix
Merge pull request #4412 from jdfreder/traitlet_notify_fix

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).