Add a default desktop background image

Change-Id: Iff9ef896ac984ced5e1869dc59c1f639ca0c9f6f
Reviewed-on: http://gerrit.dmdirc.com/681
Reviewed-by: Shane Mc Cormack <shane@dmdirc.com>
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>

src/com/dmdirc/addons/ui_swing/plugin.config

@@ -36,5 +36,5 @@ defaults:
     hideEmptyTopicBar=false
     textpanebackground=
     textpanebackgroundoption=STRETCH
-    desktopbackground=
-    desktopbackgroundoption=STRETCH
+    desktopbackground=dmdirc://com/dmdirc/res/background.png
+    desktopbackgroundoption=CENTER