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/pointerWatcher.js

@@ -8,7 +8,7 @@ const GnomeDesktop = imports.gi.GnomeDesktop;
 const Shell = imports.gi.Shell;
 
 // We stop polling if the user is idle for more than this amount of time
-const IDLE_TIME = 1000;
+var IDLE_TIME = 1000;
 
 // This file implements a reasonably efficient system for tracking the position
 // of the mouse pointer. We simply query the pointer from the X server in a loop,