Allow nesting individual clips

This might seem like an odd request but at Sketch we would love to be able to create a nested clip with only a single clip selected. This would help us for a couple of reasons:

1: We nest clips recorded on multiple different display sizes as a way of cropping footage down and "resetting" the zoom level to 100% — so we can all share the same presets and video actions to apply on the "outside" of the nested clip.

2: Sometimes we just want to establish a nested clip that we'll add more stuff too later.

Currently we're splitting clips just to turn them into 2 so that we can select them both, nest them, and then get rid of the split afterwards. It would be nice to not have to do this sort of workaround.