TerminalPanel: Disconnect destroyed signal in destructor
Otherwise, when QObject cleanup, which runs after our destructor, destroys the terminalPart, it would call back into us when we're already half-destroyed.
This trips an assert in Qt "Called object is not of the correct type (class destructor may have already run)"
The assert was added in Qt 6.3.
Edited by Kai Uwe Broulik