Add the possibility to view hidden fields on form preview

Currently, when you want to check what hidden fields are set on a form and how they are set, the only possibility is to edit the form, which create a draft.

We would need to have a "show hidden fields" toggle on the form preview pane to avoid this.

Combined with this other idea, this would enable to stop inadvertent creation of tens, even hundred of landing page drafts when people just review the form and hit "approve and close", just because they are used to it.

-Greg