Pyface Color Class and Trait Type

[corranwebster]

This builds on #520 to create a Color class and corresponding PyfaceColor trait to represent a color in a toolkit independent way. It also includes a ColorDialog class built on top of these classes.

Together with #520 this resolves #476.

This should not be merged until #520 is merged. This requires features from Traits 6.1.

[ievacerny]

Most of this looks good. There's potentially a bug with wx dialog and then some minor comments re typos and missing tests.

[codecov-commenter]

Codecov Report

Merging #534 into master will increase coverage by 2.03%.
The diff coverage is 85.39%.

@@            Coverage Diff             @@
##           master     #534      +/-   ##
==========================================
+ Coverage   37.08%   39.11%   +2.03%     
==========================================
  Files         470      485      +15     
  Lines       26027    26788     +761     
  Branches     3961     4075     +114     
==========================================
+ Hits         9652    10479     +827     
+ Misses      15948    15861      -87     
- Partials      427      448      +21     

Impacted Files	Coverage Δ
pyface/__init__.py	82.60% <ø> (ø)
pyface/ui/null/color.py	0.00% <0.00%> (ø)
pyface/ui/qt4/init.py	62.50% <33.33%> (ø)
pyface/font_dialog.py	50.00% <50.00%> (ø)
pyface/ui/qt4/workbench/split_tab_widget.py	12.47% <50.00%> (ø)
pyface/ui/wx/color_dialog.py	51.85% <51.85%> (ø)
pyface/ui/wx/font_dialog.py	54.54% <54.54%> (ø)
pyface/qt/QtGui.py	81.48% <58.33%> (-9.00%)	⬇️
pyface/qt/QtNetwork.py	75.00% <60.00%> (ø)
pyface/qt/QtOpenGL.py	75.00% <60.00%> (ø)
... and 63 more

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[corranwebster]

Pending CI passing, this should be ready for re-review.

[ievacerny]

Qt dialog now works well, but wx dialog still doesn't

[corranwebster]

I think this is ready for one more go-around with review.

[ievacerny]

I'm still having issues with wx dialog window freezing, but I don't think it's related to this PR. I'm seeing the same behaviour with other wx dialogs as well (using both 4.0.7 and 4.1.0)

Another thing is that wx dialogs for me appear without "OK" or "Close" buttons, the only way to close the dialog is via system's "x" button, which behaves differently between qt and wx (see in-code comment). The same dialog without buttons appears using TraitsUI color editor, so again, I don't think it is related to this PR specifically, but maybe it's something that can be fixed here.
Edit: Here's the issue I opened for ColorEditor_demo.py enthought/traitsui#946. I get the same dialog using get_color function from this PR.

Note: if nobody else is seeing these issues, it might be that the state of GUI machinery might be messed up on my computer. In that case please ignore the comments.

[ievacerny]

Closing the dialog via system's "x" button returns the color that was passed to this function when using wx. With qt it returns None as expected.

[corranwebster]

Everything in here has now been pulled out into a separate PR.