Implement logic for adding/removing notifiers for a given ObserverGraph

[kitchoi]

Closes #1021
Item 6 of #977

This PR adds a function add_or_remove_notifiers for adding/removing notifiers for a given ObserverGraph.

• The IObserver interface is expanded to support add_or_remove_notifiers.
• Added a INotifier interface to support add_or_remove_notifiers. The TraitEventNotifier (in Add notifier for wrapping user handler for the observer framework #1000) and ObserverChangeNotifier (in Add notifier for maintaining nested notifiers in the observer framework #1007) implement the INotifier interface introduced here. It is possible that this PR might need changes per review outcome of Add notifier for wrapping user handler for the observer framework #1000 and Add notifier for maintaining nested notifiers in the observer framework #1007. The formal registration can happen later... or not happen at all.

Checklist

• Tests
• Update API reference (docs/source/traits_api_reference): Things here are still private. Module names are preceded with an underscore.
• Update User manual (docs/source/traits_user_manual): Things here are still private.
• Update type annotation hints in traits-stubs: Nothing here are trait-ed

[codecov-io]

Codecov Report

Merging #1023 into master will increase coverage by 3.65%.
The diff coverage is 97.34%.

@@            Coverage Diff             @@
##           master    #1023      +/-   ##
==========================================
+ Coverage   73.05%   76.70%   +3.65%     
==========================================
  Files          51       61      +10     
  Lines        6514     6698     +184     
  Branches     1309     1294      -15     
==========================================
+ Hits         4759     5138     +379     
+ Misses       1363     1215     -148     
+ Partials      392      345      -47     

Impacted Files	Coverage Δ
traits/api.py	100.00% <ø> (+9.67%)	⬆️
traits/observers/_i_notifier.py	0.00% <0.00%> (ø)
traits/trait_base.py	69.23% <ø> (+5.65%)	⬆️
traits/trait_dict_object.py	94.04% <ø> (+18.59%)	⬆️
traits/trait_set_object.py	95.45% <ø> (+29.22%)	⬆️
traits/trait_types.py	72.15% <94.33%> (+0.50%)	⬆️
traits/has_traits.py	72.40% <94.44%> (+1.16%)	⬆️
traits/observers/_testing.py	98.24% <98.24%> (ø)
traits/trait_list_object.py	97.54% <99.43%> (+23.25%)	⬆️
traits/ctrait.py	71.07% <100.00%> (+12.51%)	⬆️
... and 35 more

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[corranwebster]

This looks good to me. I recall that I looked over the code on this from the POC PR, so I don't have any issues with the implementation.

[kitchoi]

Thank you @ievacerny and @corranwebster!
Merging...