Fixer for list feature.

[scofalik]

List feature is a complicated one, because it needs some precise model structure. Some model states, connected with lists, are incorrect (even though they are correct with schema, etc.). For example, a list cannot start with indented element:

<listItem indent=2>Foo.</listItem>
<listItem indent=3>Bar.</listItem>

We "are able" to created such model structure even without writing any complicated code. For instance, we could wrap part of a list in another element, or copy+paste it.

For this reason, we need some kind of fixer for list feature, that would fix those wrong structures:

1. It would be fired with high priority on each change event.
2. If incorrect structure is noticed, it will change it -- probably without using Batch API - the change has to have an immediate impact and probably should not be seen by other features (undo, OT, etc.).

This issue is to discuss what kind of incorrect list feature model structure we may encounter and how the fixer should look like.