ech layers can be used in all other topics

This PR adresses all use cases in https://github.com/geoadmin/mf-geoadmin3/issues/886

We can now search and add all 'ech' layers in all other topics. Also, on topic change, the layers are only removed if they are not part of the new topic and the ech topic.

__ATTENTION__: This needs https://github.com/geoadmin/mf-chsdi3/pull/651 to work. Keep that in mind when testing (adapt your API_URL in the rc_file to my account if you want to test it)