fix[close #59]: Creating subsystem does not show anything and just freezes

[mirkobrombin]

At first glance, I found this looks like just a typo:

but after further investigation, I found more issues, like:

• the new Subsystem object's enter_command variable not getting populated, making the console unusable
• the Subsystem's create function not properly handle arguments when apx-gui is running inside a container
• the result of the creation, not being used at all, making the error dialog never appear if anything goes wrong

plus some minor issues in typing:

• run_vte_command was declaring the return as a tuple of a bool and string, but nothing was getting returned, anyway I changed it to just a bool and implemented it using the Vte.spawn_sync result
• __on_create_clicked had an inherit create_subsystem method for no reason, we are not inside an async method here, Vte is handling it for us, no need to have a second function
• the NewSubsystem variable should follow the snake case formato, so I renamed it to new_subsystem, same for btn_startstop that is now btn_start_stop

I also added some prints in the subsystem creation process so a users can provide the command used to create the subsystem, as long as the output and result for the creation itself.