Further work on animated GIF export: XDG Pictures

Using XDG's user dir settings to determine where pictures are
stored for a user (e.g., "~/Pictures" -- used as a fallback).
May be overridden using "--exportdir".

Also, while I was updating some docs, replace references to
"Mac OS X" with "macOS", the new name of that OS these days.

docs/CHANGES.txt

@@ -8,7 +8,7 @@ http://www.tuxpaint.org/
 
 $Id$
 
-2020.July.24 (0.9.25)
+2020.July.25 (0.9.25)
   * New Features
     ------------
     * [WIP] GIF export option from Open -> Slideshow dialog.
@@ -17,6 +17,9 @@ $Id$
       * Depends on "libimagequant", from the "pngquant2" project:
         https://github.com/ImageOptim/libimagequant
         (GPL v3 or later, for Free/Libre Open Source Software)
+      * Uses XDG user dirs settings, if available, to determine
+        where to store them, or "~/Pictures", otherwise.
+        "--exportdir" option can be used to override this.
 
   * Ports & Building
     ----------------