Fix compatibility with QT_NO_KEYWORDS

Replace signals & slots keywords by Q_SIGNALS & Q_SLOTS, this ensure compatibility when QT_NO_KEYWORDS is defined in projects using Qt & VTK.

Moreover this can also fix issues when using third-party library for signals/slots like boost.

Same work has already been done in #5544 (closed) & #6727 (closed)

Closes #17583 (closed)