Replace leafpad with mousepad

Leafpad seems to have died (removed from debian).  Mousepad looks like
the proper replacement.

Thanks for all the leaves!

dotfiles/config/bmo/sensible.ini

@@ -57,6 +57,6 @@
     terminal = terminology
     terminal = st
 
-    editor = leafpad
+    editor = mousepad
     editor = gvim
     editor = gedit