Better management of member variables destruction in TerminalDisplay

Always pass a parent to a QObject constructor; this is always a good idea, as it enforces the parent/child relationship, and it also means the parent QObject will take care of deleting its children, less manual 'delete' calls.

_terminalPainter already has a TerminalDisplay parent object.

Use a std::unique_ptr to manage _printManager (it's not a QObject).