Rename PowerButtonAction enum values to Sleep and Hibernate

SuspendToRam is not entirely accurate anymore, because depending on
user configuration, Sleep can mean SuspendToRam, HybridSuspend or
SuspendThenHibernate.

Hibernate is also used around the codebase already, renaming it
from SuspendToDisk improves consistency across PowerDevil.

Boolean variables are adapted in the same way. However, where checks
for PowerManagement::canSuspend() are involved, we stick to the
canSuspend name rather than canSleep.

There are still D-Bus methods named suspendToRam and suspendToDisk
provided by the SuspendSession action. I left those untouched
because I'm not sure what users are out in the wild.