Web default handler should only set minimal mime types
Currently the implementation says: register web default handler for everything it's supports.
This can be difficult as soon as a web browser e.g. add's official support for images. In this case it would be contradictionary.
A cleaner approach would be to register this handler only for a few known mime types as we are currently implementing in Epiphany:text/html, application/xhtml+xml, x-scheme-handler/http, and x-scheme-handler/https