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/slider.js

@@ -7,7 +7,7 @@ const Lang = imports.lang;
 const St = imports.gi.St;
 const Signals = imports.signals;
 
-const SLIDER_SCROLL_STEP = 0.02; /* Slider scrolling step in % */
+var SLIDER_SCROLL_STEP = 0.02; /* Slider scrolling step in % */
 
 var Slider = new Lang.Class({
     Name: "Slider",