Color picker dialog for Color objects/PyfaceColor trait

[corranwebster]

This is the last part of #534 pulled out into its own PR.

I'm leaving this as a draft because right now I don't trust the tests being run by the modal dialog tester.

[corranwebster]

See also #534 (review) - there may be some problems with wx.

[codecov-io]

Codecov Report

Merging #711 into master will increase coverage by 1.17%.
The diff coverage is 90.23%.

@@            Coverage Diff             @@
##           master     #711      +/-   ##
==========================================
+ Coverage   39.76%   40.94%   +1.17%     
==========================================
  Files         493      510      +17     
  Lines       27259    27882     +623     
  Branches     4140     4225      +85     
==========================================
+ Hits        10840    11415     +575     
- Misses      15944    15992      +48     
  Partials      475      475              

Impacted Files	Coverage Δ
pyface/ui/qt4/data_view/data_view_item_model.py	55.46% <35.71%> (+5.46%)	⬆️
pyface/ui/wx/color_dialog.py	53.57% <53.57%> (ø)
pyface/color_dialog.py	80.00% <80.00%> (ø)
pyface/ui/wx/data_view/data_wrapper.py	86.36% <86.36%> (ø)
pyface/ui/qt4/color_dialog.py	87.50% <87.50%> (ø)
...face/data_view/data_models/row_table_data_model.py	91.25% <91.25%> (ø)
pyface/data_view/i_data_wrapper.py	92.30% <92.30%> (ø)
pyface/ui_traits.py	86.11% <93.93%> (+2.32%)	⬆️
pyface/data_view/value_types/color_value.py	95.45% <95.45%> (ø)
pyface/color.py	100.00% <100.00%> (ø)
... and 43 more

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