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.

src/tuxpaint.conf

@@ -7,7 +7,7 @@
 # http://www.tuxpaint.org/
 #
 # Default distribution version last modified:
-# September 21, 2019
+# July 25, 2020
 # 
 # $Id$
 
@@ -251,6 +251,13 @@
 # savedir=~/.tuxpaint
 
 
+### Export images and animated GIF slideshows somewhere different?
+### (Uses xdg-user-dirs config setting for PICTURES, if available!)
+### --------------------------------------------------------------
+#
+# exportdir=~/Pictures
+
+
 ### Use a different language?
 ### -------------------------
 ### Note: Where the language is a known language name (e.g., "spanish")