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/closeDialog.js

@@ -11,8 +11,8 @@ const Dialog = imports.ui.dialog;
 const Main = imports.ui.main;
 const Tweener = imports.ui.tweener;
 
-const FROZEN_WINDOW_BRIGHTNESS = -0.3
-const DIALOG_TRANSITION_TIME = 0.15
+var FROZEN_WINDOW_BRIGHTNESS = -0.3
+var DIALOG_TRANSITION_TIME = 0.15
 
 var CloseDialog = new Lang.Class({
     Name: 'CloseDialog',