add support for custom modes on Wayland

Xaver Huglrequested to merge
work/zamundaaa/custom-mode into master

Depends on kwin!8534 (merged)

With this, you can add a custom mode like so:

kscreen-doctor output.1.addCustomMode.1920.1080.75000.full

or remove it again with

kscreen-doctor output.1.removeCustomMode.0

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