Add trait_added support for FilteredTraitObserver

[kitchoi]

Part of #977

This PR adds trait_added support for the observer that observes traits using a generic filter (FilteredTraitObserver).

When a new trait is added via HasTraits.add_trait, provided that the trait_added event is fired, the new trait will be tested against the filter defined in FilteredTraitObserver, and if the trait is a match, notifiers will be attached to the new trait.

Checklist

• Tests
• Update API reference (docs/source/traits_api_reference)
• Update User manual (docs/source/traits_user_manual)
• Update type annotation hints in traits-stubs

[codecov-commenter]

Codecov Report

Merging #1108 into master will decrease coverage by 1.91%.
The diff coverage is 68.56%.

@@            Coverage Diff             @@
##           master    #1108      +/-   ##
==========================================
- Coverage   76.15%   74.24%   -1.92%     
==========================================
  Files          54       79      +25     
  Lines        6493     8481    +1988     
  Branches     1263     1610     +347     
==========================================
+ Hits         4945     6297    +1352     
- Misses       1205     1806     +601     
- Partials      343      378      +35     

Impacted Files	Coverage Δ
traits/api.py	100.00% <ø> (+9.67%)	⬆️
traits/base_trait_handler.py	61.76% <ø> (ø)
traits/observers/_i_notifier.py	0.00% <0.00%> (ø)
traits/observers/events.py	0.00% <0.00%> (ø)
traits/traits.py	75.10% <ø> (-2.45%)	⬇️
traits/util/resource.py	15.25% <ø> (ø)
traits/observers/_generated_parser.py	51.96% <51.96%> (ø)
traits/trait_types.py	72.29% <88.88%> (-0.16%)	⬇️
traits/observers/parsing.py	95.34% <95.34%> (ø)
traits/observers/_trait_event_notifier.py	96.61% <96.61%> (ø)
... and 59 more

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[mdickinson]

LGTM