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/en/html/README.html

@@ -171,7 +171,7 @@ version
     <hr size=1 noshade>
 
 
-    <h3>Mac OS X Users</h3>
+    <h3>macOS Users</h3>
     <blockquote>
       Simply double-click the "Tux&nbsp;Paint" icon.<p>
     </blockquote>
@@ -825,7 +825,7 @@ version
 
 	<dt>Printer Settings</dt>
 	<dd>
-	  <p><i>(Windows and Mac&nbsp;OS&nbsp;X)</i></p>
+	  <p><i>(Windows and macOS)</i></p>
 
 	  <p>By default, Tux&nbsp;Paint simply prints to the default
 	  printer with default settings when the 'Print' button is
@@ -998,7 +998,7 @@ version
     e.g.: <code>"C:\Documents and Settings\<i>(user&nbsp;name)</i>\Application
     Data\TuxPaint\saved\</code>"</dd>
   
-  <dt>Mac OS X</dt>
+  <dt>macOS</dt>
   <dd>Inside the user's "<code>Library</code>" folder:
     <code>"/Users/<i>(user&nbsp;name)</i>/Library/Application Support/Tux&nbsp;Paint/saved/</code>"</dd>
   
@@ -1052,7 +1052,7 @@ version
 
   <h2>Doing it Manually</h2>
   <blockquote>
-    <p>Windows, Mac OS X and BeOS users must currently do the conversion
+    <p>Windows, macOS, BeOS, and Haiku users must currently do the conversion
     manually.</p>
 
     <p>Load a graphics program that is capable of both loading your picture