Feature: rework in-game Online Players GUI

The GUI now more clearly shows some basic information about the
server you joined, your client name (and the ability to change it),
and what players are in which company.

It also contains useful buttons to press to join companies, chat
with other people, and for admins to kick/ban people.

Additionally, renamed "advertised" to "visibility"; this has to
do with future additions, but also because it is more clear in
wording.