Xpra: Ticket #2271: hide credentials from connection strings

When connecting with a URI that contains username and password, those can end up being logged:

Attached to tcp://antoine:123456@192.168.1.5:10000/

And shown in the system tray menu.

There is a little bit of confusion between "endpoint" and "target" attributes of the connection object.

Also, the ssh code re-implements its own URI logic...

This needs cleaning up.

Done in r22412 the lazy way: the messy code that sets the "target" from the constructor arguments is unchanged, we just patch it up afterwards..

See also r22399, which made me create this ticket.