synchronize gui's debug actions with the octave core thread (bug #41139) * main-window.cc (constructor): initialize new queue with debug commands (debug_continue, debug_step_into, debug_step_over, debug_step_out, debug_quit): call queue_debug instead post octave_link-event and remove the related callback functions (queue_debug): new functin handling the queue of debug actions and posting octave_link events; (execute_debug_callback): common new callback function for debug actions * main-window.h: removed functions: debug_continue_callback, debug_step_into_callback, debug_step_over_callback, debug_step_out_callback, debug_quit_callback; new functions: queue_debug, execute_debug_callback; new variables for handling the debug queue

gnulib-hg = gnulib-hg