icon-browser: Stop connecting to ::delete-event

We can just use GtkWindow::hide-on-delete

demos/icon-browser/window.ui

@@ -140,7 +140,7 @@
     <property name="modal">True</property>
     <property name="use-header-bar">1</property>
     <property name="resizable">False</property>
-    <signal name="delete-event" handler="gtk_widget_hide_on_delete"/>
+    <property name="hide-on-close">1</property>
     <child internal-child="vbox">
       <object class="GtkBox">
         <child>