Export project

[vkbo]

Add an option to export the entire project to a application-independent plain text format. See: #839 (comment)

The output should be a folder with each document written as a markdown file in a folder hierarchy reflecting the project tree. The files should optionally be prefixed with a number so that file order can be recorded. Each file should have a name derived from the file label, and in case the document is a child document of another document, the combined name of both with a user-defined separator.

This is different from a build, and should be an export tool rather than a build. There is no building/assembling of data going on here.