Be able to design multi-level complex types from Studio

Currently it's possible to design a complex list in schemas, and then within this complex type, design multi valued fields. But it is not possible to add a complex field within it: this option has been blocked for now for performance reasons.

This is issue is about unblocking this feature, and making it possible to define complex structure without any limitations.

We have two issues:

• in terms of UI on schema pages, currently sub fields are shown in a popup, but going deeper in levels would mean a pop up on a poup on a popup, etc... => maybe show sub pages like in layout screens?
• when this is done, layouts that are automatically generated from this structure should work at any level. Current generated list widgets do know how to handle properties for lower sub levels.