Patch from Petros Siligkounas

Modernize the signal-slot syntax

    Summary:
    This patch is about modernizing the signal - slot syntax to use the function pointer method. Unfortunatelly I couldn't make this work with qOverload, although I was using GCC 9.2 with >

    Let me know if there is anything I am missing. Thank you.

    Test Plan: I applied the patch with portage and kamera installed and showed up fine. I don't have the needed hardware to further test it though.

    Reviewers: mlaurent, #gwenview

    Reviewed By: mlaurent

    Subscribers: tommo, davidre

    Differential Revision: https://phabricator.kde.org/D23905