Refactor color traits (#924)

* Refactor color traits

This combines the two color traits in Enable: ColorTrait (which is mapped)
and RGBAColorTrait (which is not), updating them so that they don't use the
old `Trait` factory and are simple TraitTypes instead.  It combines validation
so that they accept the same wide set of values, and makes use of the code from
Pyface for parsing color values and the standard color names (Pyface's values
were originally taken from Enable).  It also unifies the editors, so that both
traits have the same editors.

* Prevent circular imports in Wx color editor code.

* Apply suggestions from code review

Co-authored-by: Mark Dickinson <mdickinson@enthought.com>

* Various improvements suggested from code review.

* Fix imports.

Co-authored-by: Mark Dickinson <mdickinson@enthought.com>

docs/source/enable/overview.rst

@@ -80,7 +80,7 @@ something that doesn't draw but does respond to events, subclass
 Container
 ~~~~~~~~~
 
-All components have a :class:`Container`. They can only have a single 
+All components have a :class:`Container`. They can only have a single
 container. One component can't be contained by two objects.
 
 Whenever you request a component to redraw itself, it actually requests its
@@ -321,7 +321,7 @@ which are of interest to users:
 bgcolor
 ^^^^^^^
 ``bgcolor`` is a :class:`ColorTrait` which can be used to specify the background
-color of the component. The default value is ``"sys_window"``, which may or may
+color of the component. The default value is ``"syswindow"``, which may or may
 not match the default window background color of the GUI toolkit you are using.
 
 high_resolution

enable/abstract_window.py

@@ -66,7 +66,7 @@ class AbstractWindow(HasTraits):
 
     # The background window of the window.  The entire window first gets
     # painted with this color before the component gets to draw.
-    bgcolor = ColorTrait("sys_window")
+    bgcolor = ColorTrait("syswindow")
 
     alt_pressed = Bool(False, transient=True)
     ctrl_pressed = Bool(False, transient=True)