Extract base content model editor

[Pavel910]

Changes

This PR extracts a base content model editor "framework" (like we did with the Page Builder Editor), and makes the base editor composable. This allows us to easily create editor variations with different functionality, sharing the same core (field plugins, d&d mechanics, field layouts).

The existing content model editor was also refactored to use this new base editor, as a test to prove that the "framework" works. The entire concept revolves around composable components.

How Has This Been Tested?

Manually.