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/portalHelper/main.js

@@ -50,7 +50,7 @@ const HelperDBusInterface = '<node> \
 </interface> \
 </node>';
 
-const PortalHeaderBar = new Lang.Class({
+var PortalHeaderBar = new Lang.Class({
     Name: 'PortalHeaderBar',
     Extends: Gtk.HeaderBar,
 
@@ -115,7 +115,7 @@ const PortalHeaderBar = new Lang.Class({
     },
 });
 
-const PortalWindow = new Lang.Class({
+var PortalWindow = new Lang.Class({
     Name: 'PortalWindow',
     Extends: Gtk.ApplicationWindow,
 
@@ -282,7 +282,7 @@ const PortalWindow = new Lang.Class({
     },
 });
 
-const WebPortalHelper = new Lang.Class({
+var WebPortalHelper = new Lang.Class({
     Name: 'WebPortalHelper',
     Extends: Gtk.Application,