[Hotplug Engine] Update actions at runtime and listen for added/removed

It would only detect changes to existing device actions but not adding or removing them.
Moreover, the list of actions was never updated at runtime, only the list of predicates for the device.
This causes Device Notifier to pick up added, removed, or modified device actions only on startup.

Differential Revision: https://phabricator.kde.org/D15613