Share the default timeline settings.

[na9da]

What this PR does

In v7 it used to be possible to share the default timeline state. Re-implement it in v8 so that we can capture scenes showing shadows at a particular time of the day in stories.

Note that the default timeline state is captured in the init source as a model with ID defaultTimeline instead of introducing a hardcoded config key for timeline settings.

Test me

Run through this story, observe that it captures different times of the day and the timeline playback state.

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.

[micholm]

awesome! LGTM