[FIX] mail: fix due to the changes for the app switcher whitelist

The app switcher in the enterprise branch introduces a new way to whitelist the
element in the app switcher. This commit apply this mechanism to the mail windows.

addons/mail/static/src/xml/chat_window.xml

@@ -2,7 +2,7 @@
 <templates xml:space="preserve">
 
     <t t-name="mail.ChatWindow">
-        <div class="o_chat_window">
+        <div class="o_chat_window o_in_appswitcher">
             <div class="o_chat_header">
                 <t t-call="mail.ChatWindowHeaderContent">
                     <t t-set="status" t-value="widget.status"/>
@@ -19,7 +19,7 @@
     </t>
 
     <t t-name="mail.ChatWindowsDropdown">
-        <div t-attf-class="o_chat_window o_chat_window_dropdown dropup #{open ? 'open' : ''}">
+        <div t-attf-class="o_chat_window o_in_appswitcher o_chat_window_dropdown dropup #{open ? 'open' : ''}">
             <span class="o_chat_window_dropdown_toggler dropdown-toggle" data-toggle="dropdown">
                 <i class="fa fa-comments-o"/> <t t-esc="sessions.length"/>
                 <span class="o_total_unread_counter">