Add support to use an external password manager

[rodarima]

Dillo should be able to store user/password information (and potentially others)
in a safe way, so that it is remembered in the future.

A potential solution is to add support for external password managers so we
don't need to implement it ourselves.

See: https://xn--4pv.arzinfo.eu.org/Otyugh/p/1723455843.925137
See: https://brutaldon.org/thread/112948402411922456

[rodarima]

Firefox Sync can store passwords too with the ffsclient: https://github.com/Mikescher/firefox-sync-client