treewide: Use AdwToolbarView for all windows with header bars

Part-of: <https://gitlab.gnome.org/GNOME/calls/-/merge_requests/714>

src/ui/main-window.ui

@@ -7,9 +7,8 @@
     <property name="show_menubar">False</property>
     <property name="hide-on-close">True</property>
     <child>
-      <object class="GtkBox">
-        <property name="orientation">vertical</property>
-        <child>
+      <object class="AdwToolbarView">
+        <child type="top">
           <object class="AdwHeaderBar">
             <property name="visible">True</property>
             <property name="centering_policy">strict</property>
@@ -32,7 +31,7 @@
             </child>
           </object>
         </child>
-        <child>
+        <property name="content">
           <object class="AdwToastOverlay" id="toast_overlay">
             <property name="child">
               <object class="GtkBox">
@@ -69,7 +68,7 @@
               </object>
             </property>
           </object>
-        </child>
+        </property>
       </object>
     </child>
   </template>