[Seven] Object browser widget

[deodorhunter]

This Draft PR represents the work done at Beethoven Sprint for Object Browser widget Seven integration. It's a WIP: still needs implementation and logic porting from old volto version, to be tested with a real backend that uses patternOptions for fields like Related Items, and a Seven version that allows content saving.

Note: a weird behavior was encountered when trying to reuse loaders: if you import a loader inside another loader, whole app breaks and page autoreloads, error is "Process is not defined", so it likely stems from trying to use server side code in a client side environment, maybe from plone/client?

Closes #6648