Extension.inIncognitoContext for extension tabs

Currently extension tabs in private windows always return false for extension.inIncognitoContext.

Firefox and Chrome however, always return true. And Safari considers returning false a bug, see https://github.com/w3c/webextensions/issues/1026

Orion should follow here as well and return true.

This specifically is for extension contexts opened in a new tab, not for action popups, which is not clearly specified at this moment.

Version 1.0.8 (147.1), WebKit 625.1.8

Tahoe (26)