* add min_api to manifest.toml, define WEBXDC_API_VERSION
* return an error instead of index.html if the webxdc requires a newer api
* add a test with an webxdc requiring a newer api
* update CHANGELOG
* let sending invalid webxdc fail
invalid webxdc can still be send as Viewtype::File, however
(maybe one want to discuss errors or so ;)
* clarify the supported zip compression methods