[FIX] point_of_sale: prevent crash when loading

With a recent change in web client, the point of sale could not load
anymore, because the web_client code tried to call this.menu.reflow(),
but point of sale overrided the show_application, and this.menu is not
even instantiated.

Since the point of sale actually want a barebone web client, instead of
using the full web client, removing some of its features, and then
fixing the problems each time a change in web_client breaks it, this commit
ensure that the point of sale is started with just what it needs

addons/point_of_sale/views/pos_templates.xml

@@ -31,7 +31,7 @@
 
         <script type="text/javascript" id="loading-script" t-raw="init">
             odoo.define('web.web_client', function (require) {
-                var WebClient = require('web.WebClient');
+                var WebClient = require('web.AbstractWebClient');
                 var web_client = new WebClient();
 
                 web_client._title_changed = function() {};