use the placeholders as custom variables in the pdf ticket output

[moritzlerch]

Hey,

I would like to replace some question answer from the booking process with some specific content and put it as a text variable (like a normal question) into a ticket template. Is there any way of implementing this within the plugin as an option to also register the replacement as a pretix.base.signals.layout_text_variables?

(for context: I've got a question about the food preference and the users can select one of the provided dishes from a list of dishes. Now I don't want to output the name of the dish directly but want to have a replacement string there instead.)

[rixx]

Also providing question answers as ticket layout content sounds like a good extension to this plugin, and I'd be happy to merge a PR to that extent!