[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.
1 job for master in 15 minutes and 15 seconds (queued for 46 minutes and 35 seconds)
Status Job ID Name Coverage
  Build
passed #64409
linux kf5-qt5 qt5.12

00:15:15

0.0%