Add text-only prompt element for console-ui

[quintesse]

This issue is about creating a prompt element for the console-ui that only displays text and doesn't ask the user for input.
This way a developer can introduce "sub-headers" (= text-only blocks between prompt elements) in front of input elements that will be displayed automatically (and also removed automatically when cancelling) without having to manage the prompt headers list manually.

Right now a developer can add sub-headers by manually adding strings to the list of headers between calls to prompt.prompt(), but the new prompt() method introduced in #1132 doesn't allow this. So we needed to come up with an alternative, which is this issue.