make copying figures to clipboard work (bug #44866) * Figure.cc (Figure::copy_figure_callback): Just print the figure to a file here, then call octave_link::copy_image_to_clipboard. (delete_file): Delete unused static function. * main-window.h, main-window.cc (main_window::copy_image_to_clipboard): New function. (main_window::construct_octave_qt_link): Connect octave_qt_link::copy_image_to_clipboard_signal signal to main_window::copy_image_to_clipboard slot. * octave-qt-link.h, octave-qt-link.cc (octave_qt_link::do_copy_image_to_clipboard): New function. (octave_qt_link::copy_image_to_clipboard_signal): New signal. * octave-link.h (octave_link::do_copy_image_to_clipboard): New pure virtual function. (octave_link::copy_image_to_clipboard): New static funtion.