add ``exclude_from_nav`` metadata in child items

[terapyon]

[mister-roboto]

@terapyon thanks for creating this Pull Request and help improve Plone!

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass.

Whenever you feel that the pull request is ready to be tested, either start all jenkins jobs pull requests by yourself, or simply add a comment in this pull request stating:

@jenkins-plone-org please run jobs

With this simple comment all the jobs will be started automatically.

Happy hacking!

[terapyon]

@jenkins-plone-org please run jobs

[terapyon]

@jenkins-plone-org please run jobs

[tisto]

@terapyon can you elaborate your use case to include exclude_from_nav in the metadata? I am not opposed to this feature, we just have to be careful that the metadata does not become too heavy.