3.0: [UX] area label

## The problem to solve

There is no way (at least I haven't found a reliable one) to give an area label different from "Add content" in page edit mode. From what I understand the area label can only change if it contains one only widget (the area label becomes "Add {wdiget.label}").
And while this is not an issue for a smaller projects, it doesn't scale well with bigger projects having complex nested area and widget structure.

## Proposed solution
Introduce an option for setting area label on definition level (field option) which can be additionally overridden on template level (e.g. `{% area ... with { label: 'My label' } %}`)

Start by tracing area-label handling in page edit mode, including field definitions and the `{% area ... %}` template entry point. The work is complete when an area label can be set at definition level and overridden on the template level with a custom label.

Written by the indexing model from the issue text.