queue and synchronize mutliple commands executed fro history (bug #39723) * main-window.cc(constructor): Initialization of new class variables for the command queue, its mutex and a counting semaphore for handling the queue; (destructor): delete command queue; (execute_command_in_terinal): only call new function for queuing the command; (run_file_in_terminal): checking the path and building the cresulting ommand is done here and not in the callback, the command is finally queued (queue_command): new function for queuing commands and posteing the event for the execution callback if the callback is not already active; (execute_command_callback): no parameter because working on the command queue, reposting own event if the queue si not empty, releasing a semaphore if empty; (run_file_callback): removed, code is in run_file_in_terminal now * main-window.h: execute_command_callback without parameters, new function queue_command, new variables for the command queue, its mutex and for a semaphore handling the queue