Fix vocabulary widget import when in a sub widget

Vocabulary widgets are not imported correctly when in a subwidget.

Steps to reproduce:

• create subfield of a complex field in a document schema
• generate the layout for this field
• edit the corresponding widget to make it use a vocabulary widget type and chose the vocabulary
• then import this layout into view and edit layouts.

=> the corresponding widget in the view layout has lost its vocabulary property (the edit layout is ok)