If a layer priority sits between the priorities of two layers in the
timeline, i.e. it references a gap in the timeline's layers, then
ges_timeline_append_layer will never fill this gap and create the
desired layer, so the edit in timeline-tree would loop forever. So a
check was added to avoid this.

This would be a usage error, but a user can reasonably end up with a gap
in their layers if they remove a layer from the timeline.

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