[renderjs_ui] Refactor Header gadget

-  does not rely on undefined (because of possible change nexedi/renderjs!10)
-  added comments
-  simplified logic and refactoring for obviousness

bt5/erp5_web_renderjs_ui/PathTemplateItem/web_page_module/rjs_gadget_erp5_header_js.xml

@@ -230,7 +230,7 @@
             </item>
             <item>
                 <key> <string>serial</string> </key>
-                <value> <string>961.19028.47293.9045</string> </value>
+                <value> <string>961.62417.49864.24661</string> </value>
             </item>
             <item>
                 <key> <string>state</string> </key>
@@ -248,7 +248,7 @@
                     </tuple>
                     <state>
                       <tuple>
-                        <float>1502270945.98</float>
+                        <float>1504876524.45</float>
                         <string>UTC</string>
                       </tuple>
                     </state>