Bump to 0.9.24; improve usage output

Bumping to 0.9.24; going to add a new feature momentarily.

Improved usage output (e.g., that of "tuxpaint --help"); broke
the options into groups that match the tabs in Tux Paint Config.,
added some missing options.

docs/CHANGES.txt

@@ -8,7 +8,7 @@ http://www.tuxpaint.org/
 
 $Id$
 
-2018.Oct.7 (0.9.23c)
+2018.Dec.18 (0.9.24)
   * Bug Fixes
     ---------
     * Correct issue that prevented Tux Paint from lauching on Mac OS X 10.7.
@@ -31,6 +31,8 @@ $Id$
     * Mended link to MinGW/MSYS instructions at John Popplewell's website.
       (h/t "sechidis")
 
+    * Improved and expanded usage info (e.g., "tuxpaint --help" output)
+
   * Misc
     ----
     * Verbose debugging, and "DEBUG_PRINTF()" macro now available;