launch_daemon: Refactor handling of ExternalEventSources.

* Refactor ResolveExternalEvent to actually return the resolved event,
not just whether or not one was found, and then skip resolution
in TriggerExternalEvent and ResetStickyExternalEvent, which now
should only be passed ExternalEvents.

* ExternalEventSources now store destination Events, not Jobs,
following on that refactor.

* The second variant of ResolveExternalEvents is dropped,
and instead the Register/Unregister functions are implemented.

* Trigger and ResetSticky are now done in ExternalEventSource,
which now keeps track of whether it has been sticky-triggered
or not, though it does not use this information yet.

These changes should not affect behavior, they largely constitute
a reorganization (though some TODOs are resolved.)

Change-Id: I46a51cac0edb90e90b154ef9c94791cb7a1aad94
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4509
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

launch_daemon: Added basic logging facility

* The daemon now stores many events in am internal log.
* You can use "launch_roster log" to retrieve it.

launch_daemon: Check requirements before launching a job.

* They were only launched, but we didn't check if we can actually
launch them.

BLaunchRoster::ResetStickyEvent() added.

* Used to unflag sticky events that already have been triggered once.

launch_daemon: Implemented sticky events, and registration.

* Sticky events are events that keep their signal raised, ie. even if
a job is initialized afterwards, it will still be triggered.
* Consolidated naming for external events.
* Events are now registered once they are actually being used. This
allows them to allocate the resources they need to do their thing.

launch_daemon: launching on demand now actually works.

* Was broken in two ways: if only the shortcut "on_demand" was used,
the event didn't get created at all due to a bug in
Events::AddOnDemand().
* Furthermore, _LaunchJob() always triggered a demand, but it should
only do this when not called from a target.

launch_daemon: Added support for external events.

* Other apps can register events, and the launch_daemon can act on them
when they are triggered.

launch_daemon: Fixed job event trigger.

* Always check for events before launching a job unconditionally.
* Correctly check for demand before launching a job.

launch_daemon: Outlined basic event mechanism.

launch_daemon: Added support for external events.

* Other apps can register events, and the launch_daemon can act on them
when they are triggered.

launch_daemon: Fixed job event trigger.

* Always check for events before launching a job unconditionally.
* Correctly check for demand before launching a job.

launch_daemon: Outlined basic event mechanism.