suppress all X11 and Qt error messages by default * octave-gui.cc (fdstderr): Delete variable and all uses. (message_handler): Don't print anything. (octave_start_gui): Check OCTAVE_SHOW_GUI_MESSAGES environment variable to decide whether to install message_handler to suppress messages. * __init_fltk__.cc (xerror_handler): New function. (plot_window::plot_window): Check OCTAVE_SHOW_GUI_MESSAGES environment variable to decide whether to install xerror_handler to suppress messages.

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