Define externally accessible contants with 'var' instead of 'const'

Just as we did with classes, define other constants that are (or
may be) used from other modules with 'var' to cut down on warnings.

https://bugzilla.gnome.org/show_bug.cgi?id=785084

js/misc/history.js

@@ -5,7 +5,7 @@ const Signals = imports.signals;
 const Clutter = imports.gi.Clutter;
 const Params = imports.misc.params;
 
-const DEFAULT_LIMIT = 512;
+var DEFAULT_LIMIT = 512;
 
 var HistoryManager = new Lang.Class({
     Name: 'HistoryManager',