handle removed children category container

[mariusheine]

Imagine you have a column with dynamic amount of columns.
When you decrease the column count the children container for the last column gets removed.
If that container had elements inside then these elements stay in the schema node as invisble artifacts.

If you increase the column count again these artifacts are again visible.

Proposal for handling:

• delete the nodes of the deleted children container
• move the nodes of the deleted children container to another still existing container

[mariusheine]

I think we should go with deleting the children because it is way easier as former doesn't easily know what other containers might be existing.