Strip viewer and related widgets.
The classes in this file implement the StripViewer shown in the top two thirds
of the main Pynche window. It consists of three StripWidgets which display
the variations in red, green, and blue respectively of the currently selected
r/g/b color value.
Each StripWidget shows the color variations that are reachable by varying an
axis of the currently selected color. So for example, if the color is