This patch fixes two things:

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).