Improve dbus API

[bublath]

Hi,
I'm trying to create a module that uses the Dbus API to expose Signal for my favorite home automation system (FHEM), which is Perl based.
Work pretty good so far. Great work from the author.

I realized that the dbus interface could use some enhancements.

1. I already created code that retrieves the GroupIDs and Names (and caches them), so my interface can be used to send messages to groups just by using their name. I wonder if that functionality can't be moved inside signal-cli
2. Contact lists: I would love to use contact names instead of the phone numbers to send messages. I wanted to use the same approach as for the groups, but if I'm not wrong, there is no API to retrieve the contact list, so I would probably keep my own cache when people are setting contact names. I would love to at least have an API to retrieve the contacts - again, even better would be a translation in signal-cli

I'm just at the beginning, but I'm pretty sure there is more functionality that could be exposed here.

Best regards.