Deprecate Color, RBGColor and Font traits

[ievacerny]

Closes #728

Color, RBGColor and Font traits are now deprecated with the recommendation to use traitsui instead.

Updated API references and docstrings. Description and Default Value sections seemed out of date (and sphinx was complaining) so I just removed them. Removed all references to these traits from User manual as well.

Also updated one example that uses RGBColor.

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-io]

Codecov Report

Merging #1022 into master will increase coverage by 1.95%.
The diff coverage is 97.32%.

@@            Coverage Diff             @@
##           master    #1022      +/-   ##
==========================================
+ Coverage   73.05%   75.01%   +1.95%     
==========================================
  Files          51       54       +3     
  Lines        6514     6488      -26     
  Branches     1309     1283      -26     
==========================================
+ Hits         4759     4867     +108     
+ Misses       1363     1251     -112     
+ Partials      392      370      -22     

Impacted Files	Coverage Δ
traits/api.py	90.32% <ø> (ø)
traits/trait_base.py	68.83% <ø> (+5.25%)	⬆️
traits/trait_dict_object.py	75.44% <ø> (ø)
traits/trait_set_object.py	67.54% <ø> (+1.32%)	⬆️
traits/trait_types.py	72.31% <92.59%> (+0.66%)	⬆️
traits/trait_list_object.py	97.51% <97.69%> (+23.22%)	⬆️
traits/ctrait.py	71.07% <100.00%> (+12.51%)	⬆️
traits/has_traits.py	72.75% <100.00%> (+1.50%)	⬆️
traits/observers/_i_observer.py	100.00% <100.00%> (ø)
traits/observers/_named_trait_observer.py	100.00% <100.00%> (ø)
... and 17 more

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[mdickinson]

LGTM

[corranwebster]

LGTM