Use Popen to start virtual keyboard

Before, the virtual keyboard was started by appending `&` and using `check_call` with the `shell=True` argument. A cleaner and more secure method is to use `Popen` directly.

Note that I have not tested calling the virtual keyboard with `Popen` on my computer because I don't have any virtual keyboards installed. It should start the virtual keyboard in another process and continue without waiting for the keyboard to terminate.