NgspiceAsync refactorization
Simulation process  refactorization
ConsoleOutputWindow refactorization.

Allow set title and add docstring for all functions
Show simulation errors in a separate window.

Errors presented by ngspice are sometimes long and they cannot be shown in the info bar.
Remove on_gear_button_clicked function.

It was declared for debugging purposes long time ago and it is no longer necessary.
Remove debugging print statements
Simulation reported it was failing always
Fail only if stderr contains "Error:".

Function was returning error in some situations when simlation was succesful but ngspice gave a warining. Ngspice writes in stderr not only errors but warnings and information notes.
Convert to unicode axis label because some magnitudes are represented as unicode characters