kwin/effects/cube: Add support for per-output virtual desktops

Hynek Schlindenbuchrequested to merge
schlndh/kdeplasma-addons:work/per-output-virtual-desktops into master

See kwin MR for more details.

It fixes the following issues:

  • Wayland: fixes switching desktop via switch desktop shortcuts instead of arrow keys:
    • As far as I can tell, this doesn't work on X11.
    • previously: it switched desktops on all screens in the cube effect, even if that didn't end up happening.
    • now: it affects the correct screens.
  • show correct desktop on each screen:
    • previously: the active screen's desktop was shown on all screens.
    • now: each screen shows its active desktop.

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