Add vnd.tiddlywiki-multiple to the editor type dropdown

[pmario]

• This PR adds vnd.tiddlywiki-multiple to the editor type dropdown, because I can not remember the exact name. Having it in the dropdown makes it easy.

• It also contains the German version + a fix to make the text shorter

[github-actions]

Confirmed: pmario has already signed the Contributor License Agreement (see contributing.md)

[Jermolene]

Hi @pmario I understand the motivation, but I worry that it makes the dropdown harder to use for most everyday users. Arguably there are already too many entries (for example, image/x-icon is not very useful, and text/x-tiddlywiki should be moved into the classic tools plugin).

So, I'd be happy to proceed as long as we remove/move the two items I mentioned, and then place text/vnd.tiddlywiki-multiple in the "Developer" section of the dropdown.

[Jermolene]

Thanks @pmario