GUI: no more extra command queue for debug commands * main-window.cc (constructor): remove initializations for debug queue; (debug_continue,debug_step_into,debug_step_over,debug_step_out,debug_quit): queue new debug commands into common queue; (queue_debug): removed obsolete function; (execute_debug_callback): move this function into the execute method of the new octave_cmd_debug class; * main-window.h: remove obsolete prototypes and class variables * octave-cmd.cc (octave_cmd_debug::execute): execute method of new class for debug commands; (octave-cmd.h): changed private into protected for inheritance, new class octave_cmd_debug derived from octave_cmd_exec