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/misc/extensionUtils.js

@@ -11,13 +11,13 @@ const Gio = imports.gi.Gio;
 const Config = imports.misc.config;
 const FileUtils = imports.misc.fileUtils;
 
-const ExtensionType = {
+var ExtensionType = {
     SYSTEM: 1,
     PER_USER: 2
 };
 
 // Maps uuid -> metadata object
-const extensions = {};
+var extensions = {};
 
 /**
  * getCurrentExtension: