Preferences, singletons

Create a Preference dialog for fonts and audio output devices

Turn Preferences and SoundOff into singletons.  No matter how many times
you request a new one, the same instance is returned.

Stop using singals on the parent() to access other instances, such as
sound and Preferences.

ui/display.css

@@ -1,5 +1,5 @@
 body {
-	font-family: "opendyslexic", sans-serif;
+	font-family: "{readerFont}", sans-serif;
 }
 hr {
 	height: 2px;
@@ -18,6 +18,6 @@ p, li {
 	font-size: 24px;
 }
 p.phonetic {
-	font-family: "Gentium", sans-serif;
+	font-family: "{phoneticFont}", sans-serif;
 	font-size: 40px;
 }