Add a new tool, 'gnome-shell-extension-prefs', which can configure extensions

A new tool, 'gnome-shell-extension-prefs' can load a new entry point from
extensions, 'prefs.js', which has an entry point to return a GTK+ widget.
This allows extensions to have their own preferences dialog, without each
extension needing to ship its own Python script and .desktop file.

https://bugzilla.gnome.org/show_bug.cgi?id=668429

js/misc/config.js.in

@@ -10,3 +10,7 @@ const GJS_VERSION = '@GJS_VERSION@';
 const HAVE_BLUETOOTH = @HAVE_BLUETOOTH@;
 /* The system TLS CA list */
 const SHELL_SYSTEM_CA_FILE = '@SHELL_SYSTEM_CA_FILE@';
+/* gettext package */
+const GETTEXT_PACKAGE = '@GETTEXT_PACKAGE@';
+/* locale dir */
+const LOCALEDIR = '@datadir@/locale';