Add line to clarify Form view modes’ distinction

Description

The Form container has 3 view modes:

• On-Page Editing
• All Properties
• Form submissions

The first two are used to change the form, while the last is used to manage submissions. They are displayed similarly in the dropdown.

In the Commerce Catalog page, an entry can have several view modes, similar to Forms. Here, a line is used to separate the edit view modes (On-Page Editing and All Properties) from other view modes.

The form view mode dropdown should also have this line because it shows that there are differences between Form Submissions from other view modes. It increases consistency between Form UI and Commerce Catalog UI.