[UX] Smarter field label displays

Describe your issue or idea

This is a follow-up to https://github.com/backdrop/backdrop-issues/issues/317#issuecomment-3904... from @quicksketch:

I certainly don't want a label on images most of the time, or on the fields @JayaJayanth listed above, but if I have a bunch of arbitrary (short) textfields, I need the label to tell the difference between them.

My suggestion here would be to:

• Allow each field type to provide a default in hook_field_info() as default_label_display.
• Specify <hidden> as the default for long texts, images, and list fields.
• Any field that does not provide a default_label_display should use inline.

If we later wanted to provide a default_label_form option, that could apply to the add/edit forms instead of on display, but we don't have label settings on those forms currently.