Manifest keys cleanup

[carlosjeurissen]

To start with a clean slate, this PR removes manifest keys not supported in all browsers and not agreed upon by all vendors. We can then add some of them back once we come to agreements. See #121

1. Removal of browser-specific manifest key-names: chrome_settings_overrides, chrome_url_overrides, incognito, omnibox. See Proposal: Make extension APIs browser neutral #113
2. Removal of options_page. See Proposal: deprecate options_page manifest property #108
3. Removal of author / developer metadata: author, homepage_url, developer. See: Inconsistency: Streamline homepage_url, author and developer keys #67
4. Removal of action-related keys: action, browser_action, page_action, sidebar_action. See: Document browser_action and page_action or document action (Manifest v3) in the manifest-schema #50
5. Removal of keys not supported by all browsers. version_name, user_scripts, theme, storage, externally_connectable, devtools_page. These can be reintroduced even when they are not supported by all browsers. Yet right now it's unknown whether there is an agreement on these by those who didn't implement them (yet).

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[zombie]

Some of the keys like action are probably already supported everywhere (just released Safari Technology Preview, behind a pref in Firefox Nightly), but we can fix that after it hits release.

LGTM, and sorry for the delay.