Use CallsManager in MainWindow and NewCallBox

This removes the status message at the end of the NewCallBox, since it
will be replaced by a permanent InfoBar.

As a side effect this removes also the disconnect message when a call
ends.
Fixes: https://source.puri.sm/Librem5/calls/issues/119

src/ui/new-call-box.ui

@@ -145,17 +145,5 @@
         </child>
       </object>
     </child>
-    <child>
-      <object class="GtkLabel" id="status">
-        <property name="visible">True</property>
-        <property name="can_focus">False</property>
-        <property name="wrap">True</property>
-      </object>
-      <packing>
-        <property name="expand">False</property>
-        <property name="fill">True</property>
-        <property name="padding">16</property>
-      </packing>
-    </child>
   </template>
 </interface>