Form splitting into tabs

I propose a form splitting technique into tabs. Maybe some setting in the collection that allows me to send one field to a different tab.

I have a `Posts` collection and it contains a lot of fields (meta description & keywords, excerpt, content, title, tags. category, publish date etc.).

Example yaml:

```
- name: posts
label: Posts
label_singular: Post
folder: static/content/collections/posts
create: true
slug: "{{year}}-{{month}}-{{day}}-{{slug}}/"
fields:
- {label: "Title", name: "title", widget: "string"}
- {label: "Publish Date", name: "date", widget: "datetime"}
- label: "Meta"
name: "meta"
widget: "object"
form_stack: "Meta"
fields:
- label: Meta keywords
name: meta_keywords
widget: string
required: false
- label: Meta description
name: meta_description
widget: string
required: false
hint: "If left empty we will use the excerpt instead"
```

Notice the `form_stack: "Meta"` field. In admin this will form a separate tab named "Meta" and add there all the fields under this stack. Everything without a `form_stack` field will be sent to `Main` or something. This would allow us to let that form breathe a little.