Add the ability to set a different set of landmarks and headings for the default and the admin theme

Problem/Motivation

At the moment every theme has the same set of landmarks and headings. for example in the default theme it might be useful to display only the heading within the main element with a higher number of heading levels while in the admin theme the h3 used in the navigation sidebar might be of use for some users. there might be several different needs depending on the theme. and there might be even sites with more than a default and an admin theme running.

the issue was raised during the discussion with @mgifford today

Steps to reproduce

Proposed resolution

add the option to set the headings and landmarks per default and admin theme. and for a followup "maybe" also the option to add for more installed themes.

Remaining tasks

User interface changes

API changes

Data model changes