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/animation.js

@@ -7,7 +7,7 @@ const St = imports.gi.St;
 const Signals = imports.signals;
 const Atk = imports.gi.Atk;
 
-const ANIMATED_ICON_UPDATE_TIMEOUT = 16;
+var ANIMATED_ICON_UPDATE_TIMEOUT = 16;
 
 var Animation = new Lang.Class({
     Name: 'Animation',