screenbrightnessplugin: Extract from initialstart and use in shell

This is partially an emergency fix also for #404 (closed), which was potentially introduced by !582 (merged)

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.