1) Connect the "delete_event" signal for the main window of testgtk to
gtk_main_quit, so that the application will really quit, instead of
the window just disappearing and the application still running.
2) Fix two cosmetic bugs with buttonboxes as used in the color selection
dialog and the file selection dialog. These should probably be of type
GTK_BUTTONBOX_SPREAD, not GTK_BUTTONBOX_(END|START).
