Feature: Restricting the number of subsections of a specific 'type'

[mpsonntag]

With basic cardinality rules implemented for Property -> Values, Section -> Properties, and Section -> Sections, the topic came up if we would also like to have more specific rules, especially for Section > Sections. One idea that came up is restricting the number of subsections of a specific type.

Originally posted by @achilleas-k in #361 (comment)

[jgrewe]

I do like this, I think this would be a valuable feature for defining templates.