Fix colspan for widgets handling their labels on a form layout

Steps to reproduce:

• on a form layout (for instance the edit layout of a custom document type), add a "container" widget type
• configure this widget type to "handle its label" and to be "foldable"
• add a subwidget to this container (for instance a widget of type "layout" referencing the "dublincore" layout)
• make sure the layout only defines one column

=> the container widget only takes the place of the labels column instead of taking the whole table place