After a lot of failed attempts and head banging working trying to find a way to reuse
the standard editor-switching/creation code, I've just hacked in a temporary solution
here so that users can load old files and see the old timeline instances replaced
with Dopesheet-Timelines.
Note: This is not nice code, and copies a lot of the standard initialisation code,
but it works well enough for now. We can revisit this later when the other mode changes
come along.
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