The CGroup class started out as a dumb data store, that did some
fetching of relevant data.

It then gained a more complex async operation. The lifepsan of the
CGgroup object is managed by the model, so could get deleted whilst the
runnable was running. QRunnables and non-qobjects leds to a lot of
potential problems. There was a complex mutex and a wait condition, yet
it still misses a case only solvable with yet more mutexes.

By moving the callback handling logic to the controller, we can guard
everything in a safer more Qt manner without any overhead and with
simpler code.

There is a behavioural change if you call pids whilst things are
loading, but given a signal is emitted when pids load that's fine.

This class is exported, but the header was never installed.
Whilst technically it is an ABI break it pragmantically will have no
impact whatsoever.

BUG: 430615