[FEATURE] Re-introduce mixed overlay mode for content fallback

This "feature" solves a lot of issues, but I really wanted to make sure
to cover the generic issues around this topic.

In order to understand this, we need to understand the different
logic when talking about "language fallbacks", but I don't want
to warm up how it was before.

This is how it works:
- fallbackChain: defines which page translation should be checked
when linking or resolving a page in a specific language
- fallbackType: Now that we covered all relevant cases, the naming
could be better, but here we go, it's actually defining the logic for fetching
content.
- fallbackType="strict" -> only show the content that is viable in the target language
  but this is based on "overlays". Fetch all "language=0" records, do overlays
  and remove the ones that have no overlays. However, take the ones that have no
  language parent and render them as well ("includeWithoutDefaultTranlsation")
  This is what we call "do overlays with floating".
...