Fix space accumulation between .widgetPanel and .foldableBox

The .widgetPanel class is used on each widget box, displayed in the grid layout (summary, tab design from Studio).
The foldable boxes are currently styled as a standalone box.
Since the foldable boxes are a part of the widgets library, there are css rules conflict between margin and padding from the .widgetPanel styles and the .foldableBox styles itself

We need to see if:

• the foldable box has to be displayed in a widgetPanel container, and adapt the foldableBox styles
• the foldable box is a container at a same level than a .widgetPanel container, and adapt the foldableBox styles in another way.