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/overview.js

@@ -35,7 +35,7 @@ const DND_WINDOW_SWITCH_TIMEOUT = 750;
 
 const OVERVIEW_ACTIVATION_TIMEOUT = 0.5;
 
-const ShellInfo = new Lang.Class({
+var ShellInfo = new Lang.Class({
     Name: 'ShellInfo',
 
     _init: function() {
@@ -87,7 +87,7 @@ const ShellInfo = new Lang.Class({
     }
 });
 
-const Overview = new Lang.Class({
+var Overview = new Lang.Class({
     Name: 'Overview',
 
     _init: function() {