export: simple dependencies

[erwin]

Allow simple dependencies to be specified

Example:

• News is added and published.
• the news item itself is published and exported
• the news overview page is not exported at the moment.

--> it should be possible to specify that the news overview should be exported when a news item is published

same thing is true for blog entries and blog overview pages.

In the simplest implementation, a list of 'dynamic pages' (like the home page, news overview page, etc) are specified that are always exported at every single export.

It would make the single export a bit slower, but it eliminates the need to do a (manual) full export every time a news/blog entry is added.

[erwin]

This should be seen in a broader 'export dependencies' issue