gtk-demo: make the sidebar request proper width

- 25 chars sounds about right for the texts we use
- don't use min width so we allow shrinking the widget (large text or
  small mobile devices)
- ellipsize the text instead of clipping it.

demos/gtk-demo/main-listitem.ui

@@ -10,6 +10,8 @@
         <property name="child">
           <object class="GtkInscription">
             <property name="hexpand">1</property>
+            <property name="nat-chars">25</property>
+            <property name="text-overflow">ellipsize-end</property>
             <binding name="text">
               <lookup name="title" type="GtkDemo">
                 <lookup name="item">expander</lookup>