MultiColumn doesn't remove children when it's deleted.

[hannson]

When a MultiColumn is deleted, its contents (Columns and their contents) become undeletable.

Reproduce the problem:

1. Create a MultiColumn 2 . Create 2 Columns inside

2. add TextPlugin with some content into each Column

3. Delete MultiColumn without deleting its child plugins.

   Children don't get deleted and become orphans.