inspector: Actually destroy the property editors

We were never finalizing any of the popovers used for editing
properties. Fix that.

gtk/inspector/prop-list.c

@@ -180,6 +180,8 @@ row_activated (GtkTreeView *tv,
 
   gtk_widget_show (popover);
 
+  g_signal_connect (popover, "hide", G_CALLBACK (gtk_widget_destroy), NULL);
+
   g_free (name);
 }