I updated the MR with a fourth commit that adds a structural D-Bus API change, in addition to pulling the interface out from the fixed action object into a stand-alone org.kde.ScreenBrightness.
I know it's a substantial change to review, but the D-Bus API here is genuinely an improvement (imho) over the one originally introduced in !361 (merged) and if we tweak it before the release, we won't have to deal with backwards compatibility concerns afterwards.
Unless we don't care about backwards compatibility concerns for PowerDevil actions in the first place?
I'll figure out tomorrow how to get the tests to pass. Let me know what else I can do to push this MR along.