The "Nla Strip Controls" channel is used to house the per-strip
FCurves for controlling the strip_time and influence properties.
It sits above the active action's first group, at the same level
in the hierarchy as other groups.
TODO: It looks like a dedicated FCurve channel is needed for these
control FCurves, so that we won't accidentally apply NLA mapping
or have these FCurves disabled by the path lookups failing.
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