source/signal: Missing <sdeventplus/evet.hpp>

source/signal: Implement

source/io: Implement

internal/sdevent: Build impl into object

source/event: Implement event_handler style sources

source/time: Implement

utils: Fix type of performCallback

source/base: Make callback handling generic

internal/utils: Errors now have names

source/base: Refactor out callback routine

The callback mechanism will be used by other callback methods in the
same fashion, so we can template and re-use this functionality

source/base: Support automatically configuring userdata

This makes callbacks work correctly as the systemd callback functions
need to know where to look for the object storing the real callback.

source/base: Implement moving

source/base: Make it possible to get the prepare callback

source/base: Make it possible to get the underlying sd_event_source

source/base: Only disable the event if it exists

This shouldn't happen in practice but will be needed to supporting
moving.

source/base: Take an event directly instead of sdevent

Also expose a mechanism for retrieving our internal event.

source/base: Constify

event: Add trivial functions

clock: Add class for future use

event: Allow extracting the event and sdevent impl

event: Constify operations

event: Constify sdevent

sdref: Moves should actually invalidate the reference

This also makes it possible to use in boolean expressions to check
validity.

sdref: Fix styling

sdref: Constify sdevent