Remove dependencies from custom item classes

[adeneys]

The custom item pattern requires custom item implementations to be implicitly convertible with Sitecore.Data.Items.Item. Requiring dependencies in the custom item breaks this pattern and is an indicator that the custom item has too many responsibilities. The responsibilities which require the dependencies should be encapsulated into separate classes.

We currently suffer from this problem in the WeBlog code. Many of the custom items have dependency requirements. We should refactor the custom items to extract the additional functionalities to other classes and simplify the custom items.