Allow to terminate connections by clicking their connection indicator

This is useful for debugging
Go back to login page if the connection is lost

Fixes #9.
Show which connection the indicator is mapped to

This shows little characters over the connection indicator circle to
make it easier to remember which indicator is for which connection.

A - account server
C - chat server
G - game server
Show error if connecting to the account server failed

In the future it should probably try to autoreconnect (or at least
provide a way to reconnect without restarting).