Support SIGUSR1/2 for immediate(ish) quit-and-save

Respond to a SIGUSR1 or SIGUSR2 signal by configuring itself
for autosave, saving a new file, or overwriting the current,
respectively, and sending itself a SIGTERM (which _generally_
causes Tux Paint to quit, depending on where you are in the UI).

Closes https://sourceforge.net/p/tuxpaint/feature-requests/188/

docs/en/html/README.html

@@ -17,12 +17,12 @@ version
 </h1>
 <h3>A simple drawing program for children</h3>
 
-<p>Copyright 2002-2018 by various contributors; see AUTHORS.txt<br>
+<p>Copyright 2002-2019 by various contributors; see AUTHORS.txt<br>
 <a href="http://www.tuxpaint.org/">http://www.tuxpaint.org/</a></p>
 
 <p>June 14, 2002 -
 
-				December 18, 2018</p>
+					April 3, 2019</p>
 
 </center>
 
@@ -1159,6 +1159,9 @@ version
   <li><a href="../SVG.txt">SVG.txt</a><br>
     Notes on creating SVG format vector images for use in Tux&nbsp;Paint.
 
+  <li><a href="../SIGNALS.txt">SIGNALS.txt</a><br>
+    Information about POSIX signals Tux&nbsp;Paint responds to.
+
   </ul>
 </blockquote>