core: Persistent wake-up list on disk

Currently, on Plasma Mobile, we have one daemon each application who want to wake up from sleep. Because powerdevil doesn't save wake-up list on disk, on reboot/crash the wake-up list is lost. So applications running their own daemon on background to act as bridge between powerdevil and client, as well as save the scheduled wake-ups. This MR adds persistent wake-up list to powerdevil, so wake-ups are not lost. This will eliminate the need for application daemons since powerdevil can call clients directly and provides a reliable wake-up service.