[plugins/lockdevice] Port to use logind interface
KScreenLocker does not allow unlocking the screen via the screensaver interface by design (https://bugs.kde.org/show_bug.cgi?id=425616). However it allows to do it via the logind interface, so let's use that here. While at it also refactor the code and properly track and send the state, so the other device can show an appropriate label.
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