Multiple phases can be moved to animate-in when no elements are watched

## Issue

When no elements are watched via the `data-seq` attribute, multiple steps can be moved to the `animate-in` position at the same time.
## Reproduce

Using the following HTML:

```

Navigate forwards twice in quick succession (navigation skipping), and the next two steps will both move to `animate-in`.
## Expected

Only one phase should be in the `animate-in` position at a time.
## Further

In the Sequence.js themes, we always watch at least one element so this isn't an issue. To not watch any elements seems a little hacky but developers may choose to do this for their own reasons, especially when first learning how Sequence.js works. They may not know how watched attributes work and as such, this will appear like broken functionality to them.