This is partially an emergency fix also for
https://invent.kde.org/plasma/plasma-mobile/-/issues/404, which was
potentially introduced by https://invent.kde.org/plasma/plasma-mobile/-/merge_requests/582
This MR extracts the screen brightness implementation from initialstart
to a QML plugin, and also ports the action drawer brightness slider
implementation to it. This avoids importing the powerdevil screen
brightness plugin on the lockscreen, which apparently has a race
condition that causes it to sometimes segfault.
Having a global set margins and orientation (that were calculated from the panel containment) caused a lot of issues with the way bindings were evaluated across panels, and with high coupling.
Now use properties from within containments to determine shell margins instead, which removes the dependency on other containments for measurements. This allows us to get rid of TaskPanelControls as well!
Fixes: https://invent.kde.org/teams/plasma-mobile/issues/-/issues/198
Move from a C++ library + QML plugin to a QML plugin only for simplicity, since the homescreen switching architecture will be done from Plasma, and so use of the shell library only needs to be from QML.
