WIP Starting to make a color mixer tool

Also, modularize color picker's & color selector's method of
generating a fresh version of their active/inactive buttons,
after a color has been chosen by their tool.

docs/CHANGES.txt

@@ -7,7 +7,7 @@ Various contributors (see below, and AUTHORS.txt)
 http://www.tuxpaint.org/
 
 
-2022.January.25 (0.9.28)
+2022.January.26 (0.9.28)
   * Improvements to "Paint" and "Lines" tools:
     ------------------------------------------
     * Brush spacing may now be altered within Tux Paint.
@@ -67,9 +67,6 @@ http://www.tuxpaint.org/
 
   * Improvements to Color Picking:
     ------------------------------
-    * Show a "pipette"-shaped mouse pointer when selecting a
-      color from the color palette, or the picture.
-
     * A keyboard shortcut is now available for picking colors from
       the canvas more quickly.  Hold either (left or right) [Ctrl] key
       while clicking, and the color selector option will appear.
@@ -77,8 +74,13 @@ http://www.tuxpaint.org/
       (or outside the canvas to abort).
       Closes https://sourceforge.net/p/tuxpaint/feature-requests/209/
 
+    * WIP: A new color mixer has been added.  WORK IN PROGRESS
+
+    * Show a "pipette"-shaped mouse pointer when selecting a
+      color from the color palette, or the picture.
+
     * Don't play 'bubble' paint sound when color picker or selector
-      are aborted (e.g., via their "Back" buttons).
+      (or new mixer) are aborted (via their "Back" buttons or [Esc] key).
 
   * Localization Updates:
     ---------------------