Add placeholder to Group variations

[zaguiini]

When binding an empty Paragraph, we see a nice placeholder:

When binding a Group, though, we see the "choose your variation" card:

This is OK because you're defining the content model, but the problem is that, when entering data, the same thing is shown, despite the data entry mode being used explicitly to add content, not define layout:

Done is

• When entering data, do not display the Group variation picker.