daemon: Add virtuals to DisplayBrightness for optional "native" dimming
Depends on !466 (merged). Further progress on #38 (closed).
No KWinBrightnessDisplay implementation yet, I'm hoping that @zamundaaa will add that to KWin, plasma-wayland-protocols & libkscreen.
What I would also like to see is software-based dimming for external displays by default, to reduce NVRAM writes back to an acceptable level for many years of monitor longevity. We can't do this on X11 with PowerDevil by itself, but with KWin integration on Wayland it's totally possible and a much better default.
This is meant for display devices that provide a separate dimming multiplier setting independently from the nominal brightness value. By keeping nominal brightness untouched, displays won't have incorrect brightness values in the edge case when shutting down while the display was still dimmed.
KWin will implement this interface with a new Wayland protocol.
Direct-access display brightness controls (in X11) only provide a single brightness value and are not expected to implement this.