Conditional fields: `then` option#54

Since Kirby 3.1.0, we have the option to conditionally show and hide fields using when. Would it be possible to also have a then option in the blueprints that takes field settings that are applied as soon as the when condition is met? I’m thinking of different placeholder texts based on conditions or of different textarea sizes depending on the layout needed for a condition.

Detailed discussion: https://github.com/getkirby/ideas/issues/257