Specify meaning of `homepage_url` and `developer.url` manifest keys

[carlosjeurissen]

Background

Extensions can declare two types of URLs:
homepage_url (mdn) and developer.url (mdn).

Chromium-based browsers use homepage_url for the "view extension website" feature in the extension configuration page.

Opera is claimed to have support for developer.url yet I could not find any evidence for this. The claim is it would prefer homepage_url over developer.url yet it seems developer.url is simply never used.

Firefox currently has a generic "Homepage" link in the extension configuration page and uses developer.url property with homepage_url as fallback. At the same time, it has a link next to author which links to the add-on store if it is an add-on store installed extension, else it is just text.

Moving forward

Instead of deprecating one over the other, I believe we can redefine/clarify the properties meaning.
homepage_url: homepage of the individual extension
developer.url: homepage of the author/publisher/developer in question

This would mean Firefox probably needs to swap the above mentioned fallback logic if "homepage" is supposed to be about the extension homepage. For the author link, developer.url can be used, unless for security/design reasons it should stay the add-on URL.

Browsers currently not supporting developer.url can introduce it as author/publisher/developer website link or use it as fallback for the website link in case homepage_url is missing.

Note: this has partly been discussed in #67. To keep the issue clean and make sure status labels make sense, I split off the author part for additional clarity, with a clear goal: deprecate the author field.