Stream based conversations & code signing fixes

The conversations were handled in a promise fashion using the query helper from the nodejs-assistant library. While this approach is great to get started, we needed to migrate to the much more powerful stream-based API. The conversations are currently buggier than before, but they kinda work. Using a stream-based approach also opens the way to audio conversations.

Besides the changes in conversations, some magic has also been done to code signing, Gatekeeper assessment and app notarization on the macOS build to make it work correctly on Mojave >= 10.14.5 (see: electron-userland/electron-builder#3828).

Mojave support

Xcode version on CI has been updated so that the built app will be able to work on Mojave.

Initial release!

This initial release offers only really basic functionalities, such as text only stateful conversations with the Assistant, audio responses from the Assistant and a neat UI. This initial release will mainly be used to check if auto updater works correctly, but it's still a great starting point, isn't it?