in-app-notification: compose with GtkBin instead of subclassing GtkRevealer

Note that the GtkOverlay must pass-through interactions for this to
work, meaning it's no longer possible to interact with the
notifications. I'm leaving it like this since I'll need to replace it
with AdwToast later anyways.

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

src/ui/call-window.ui

@@ -15,6 +15,9 @@
           <object class="CallsInAppNotification" id="in_app_notification">
             <property name="visible">True</property>
           </object>
+          <packing>
+            <property name="pass-through">True</property>
+          </packing>
         </child>
         <child>
           <object class="GtkStack" id="main_stack">