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/ui/osdMonitorLabeler.js

@@ -9,7 +9,7 @@ const Main = imports.ui.main;
 const Tweener = imports.ui.tweener;
 const Meta = imports.gi.Meta;
 
-const FADE_TIME = 0.1;
+var FADE_TIME = 0.1;
 
 var OsdMonitorLabel = new Lang.Class({
     Name: 'OsdMonitorLabel',