Import feature does not ask for confirmation

Problem/Motivation

The import button overrides every field in a config page without asking for a confirmation.
This makes it easy to click it by accident.

Also if no context is selected, it will simply clear every field.

Steps to reproduce

Create a config page on a multilingual site and enable Language context.
Browse to the edit form, fill the fields and save.
Browse to the edit form and click Import: the fields are now empty.

Proposed resolution

It should ask for a confirmation, similar to the "Clear values" button.

Remaining tasks

User interface changes

API changes

Data model changes