When a contact shows info from multiple sources, even the default source should have a section header

Affected version

• GNOME Contacts version: gnome-contacts-44~rc-1.fc38.x86_64
• Application provider: distribution
• Related info: Fedora 38

Steps to reproduce

1. Add a local contact with foo@example.com email and some phone number.
2. Add a Google account in Online accounts, see Contacts get populated by your Google accounts
3. Create a Google account with foo@example.com email and some home address.
4. Those two contact should collapse into a single contact view, and their data show show up together, just separated into different sections.
5. The first section display doesn't get a label. But it's not clear what it is. It changes depending on what your default address book is. It should have a label.

Current behavior

The first section is not labeled. It's not clear that in this case, the first two items are displayed from my Google account:

This is crucial especially when editing:

Expected behavior

Also label the first section, when there are multiple sections. E.g. Local Contact, Google contact, etc.

Additional information

This is a follow-up from #300 , where you can see how confused users can get when you don't expose important info to them.