Refactor both DocumentBuilders to one DocumentFactory

[JaZo]

Description

I dropped both DocumentBuilders and created a DocumentFactory that makes an ItemDocument or CollectionDocument based on the data you give it. The factory doesn't hydrate the items anymore, that's up to the ItemHydrator. It does gather the included itself based on the relations of the item(s) instead of asking the item for the items that should be included.

Motivation and context

This should improve the code quality by moving some logic to the responsible class and simplifying the Item (currently the worst rated class according to Scrutinizer).

How has this been tested?

Tested with updated unit tests.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.
• Each individual commit in the pull request is meaningful.
• I have added tests to cover my changes.
• If my change requires a change to the documentation, I have updated it accordingly.