WIP: Add autofill (using GtkPopover)

This is related to #193 and works with the latest upstream as of today (2019-03-02) with some deprecations related to JavascriptCore. And I will rewrite it using JavascriptCore since it would be sad to add a new feature with deprecations.

I'm still very much interested in seeing this through but I need as much details/mockups as possible and the more features/functionality (e.g: adding addresses & cards automatically), the more time it will take to finish. Especially that I have a full time job and can't work only on this even for a short period of time:sweat_smile:.