Slight cleanup of blind.c; new Checkerboard tool

Use enums everywhere; credits/copyright clean-up.

docs/AUTHORS.txt

@@ -7,7 +7,7 @@ Various contributors (see below, and CHANGES.txt)
 http://www.tuxpaint.org/
 
 
-June 17, 2002 - February 10, 2021
+June 17, 2002 - February 15, 2021
 
 $Id$
 
@@ -358,6 +358,11 @@ $Id$
   * Cartoon
     * "cartoon6.wav" from http://www.grsites.com/
 
+  * Checkerboard
+    * Horse gallop from
+      https://bigsoundbank.com/detail-0611-horse-gallop.html
+      Creative Commons CC0 1.0 Universal
+
   * Clone
     * https://commons.wikimedia.org/wiki/File:Sheep_bleating.ogg
       Public domain

docs/CHANGES.txt

@@ -8,7 +8,7 @@ http://www.tuxpaint.org/
 
 $Id$
 
-2021.February.10 (0.9.26)
+2021.February.15 (0.9.26)
   * New Features
     ------------
     * [WIP] Larger UI buttons
@@ -21,12 +21,16 @@ $Id$
       
   * New Magic Tools:
     ----------------
-    * Pixels
-      Draws large squares, for classic computer 'pixel art' style drawings.
+    * Checkerboard
+      Cover the entire canvas with a checkboard pattern using the
+      current color.  Drag to change the size of the squares.
 
     * Clone
       Clones (copies, via painting) part of a drawing to another position.
 
+    * Pixels
+      Draws large squares, for classic computer 'pixel art' style drawings.
+
   * Documentation updates
     ---------------------
     * Documentation is now maintained in a separate source repository,