This color class abstracts away all the possible representations of a
color in various UIs. These are color objects:

Color(9)              # red
Color('red')          # red
Color(196)            # red
Color('NEUTRAL')      # neutral (resets attributes)
Color()               # no effect

The configuration stores Color objects now, code that requires colors
doesn't have to worry about integer representations of colors.

One semantic change: when an empty color is passed as a priority color,
it will resort to an empty color instead of the neutral color. In
practice this should have the same visual effect. Test case
test_priority_color4 is adapted for this.

This commit is the result after an initial refactoring of the color code
(squashed its history into this one). @mruwek also contributed to this
refactoring, thanks for the help and the ideas to get to this point.