Let the tree view keep the focus when clicking on an item (bug #11863)

When clicking on a tree view item, the main view will open it and thus
grab the focus. This focus change may be a bit unexpected to the user,
but trivial to fix, so make the tree view keep the focus after such a
button click.

thunar/thunar-tree-view.c

@@ -770,6 +770,7 @@ thunar_tree_view_button_release_event (GtkWidget      *widget,
           else
             thunar_tree_view_action_open_in_new_window (view);
         }
+      gtk_widget_grab_focus (widget);
     }
 
   /* reset the pressed button state */