Define classes with 'var' instead of 'const'

Any symbols (including class properties) that should be visible
outside the module it's defined in need to be defined as global.
For now gjs still allows the access for 'const', but get rid of
the warnings spill now by changing it.

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

js/ui/grabHelper.js

@@ -46,7 +46,7 @@ function _popGrabHelper(grabHelper) {
 // your code just needs to deal with it; you shouldn't adjust behavior directly
 // after you call ungrab(), but instead pass an 'onUngrab' callback when you
 // call grab().
-const GrabHelper = new Lang.Class({
+var GrabHelper = new Lang.Class({
     Name: 'GrabHelper',
 
     _init: function(owner, params) {