Add a way of pausing progress in script composite loops

[SanderVocke]

Scripts make it possible to create complex building looping patterns.

But it would be nice to have a way to pause their execution along the way, e.g. to take the pressure off having to perform everything perfectly at once.

A script could have sections. A section could end in a checkpoint, where the preceding section keeps looping until the user tells it to proceed, or even choose to re-record that section.