The "what's inside what" view, from the single Application down to one field's widgets. Each
level is a real containment relationship in the code, not just a call:
Application.show_main_window() builds exactly one MainWindow
(app.py:55-63); DocumentsDock holds one DocumentWidget per open path
(documents_dock.py:26); DocumentWidget builds a viewer dock and an editor dock from the same
FieldsForm (document_widget.py:38-40) -- the [[plugins#viewer-editor-both]] surfaces.