Make the example-window.ui glade friendly
Loading the current example-window.ui in glade 3.22.0 and saving it again breaks the ui file. If one starts the example afterwards the titlebar does not longer scale properly.
To make it easy for contributors the ui file should roundtrip in glade.
Also glade throws the following warnings when starting up:
** (glade:12981): CRITICAL **: 14:59:47.343: hdy_leaflet_set_visible_child: assertion 'GTK_IS_WIDGET (visible_child)' failed
** (glade:12981): CRITICAL **: 14:59:47.343: hdy_leaflet_set_visible_child_name: assertion 'name != NULL' failed
** (glade:12981): CRITICAL **: 14:59:47.384: hdy_leaflet_set_visible_child: assertion 'GTK_IS_WIDGET (visible_child)' failed
** (glade:12981): CRITICAL **: 14:59:47.384: hdy_leaflet_set_visible_child_name: assertion 'name != NULL' failed