Enhancement: Add default value support for group fields

[estruyf]

@estruyf , one word amazing! I have been able to rebuild my troublesome about-section. A couple of things, is it possible to add an image field to fieldGroup and add a default string to a field?
When I select an image the dialog closes or when I look at the current about-section, the image is not found (that could be because it is not in the media folder).
A workaround for the default string is to create a string with one choice and I tried to use a placeholder without success.

Originally posted by @apowell656 in #176 (comment)

[bwklein]

I don't think that defaults work now for any fields in a field group.

In this video I set a default string of "Page Content" to the 'label' field in a field group. When I add that page section to the array, the label is not pre-populated with the default string.

no_default.mp4

[estruyf]

Correct, seems I myself did it wrong 🙈, it is for the group fields only indeed.

[zivbk1]

Can those defaults use placeholders like {{now}}?

[estruyf]

@zivbk1 took a bit more work, as this is something which is performed on the backend, but now the block fields can also leverage the same placeholder logic as the root fields.

[zivbk1]

Thank you!