Major lobby cleanup.

Create an object holding the handling of all netmessage types.
Display private messages sent by moderators.

Details:
Switch back the second and third argument of CreateSimpleMessage, so it relates again to the switch/object.
Reduce the dimension/nesting of the object/switch from three to two.
Hence remove the useless "standard" and "internal" value send in the text field.
Rename CreateSimpleMessage to CreateGUIMessage
Use the property "level" instead of "text" for "connect" and "disconnect".
Remove the handling of the "connect" event in lobby.js, since it is never called.
Translate the disconnect event.
Add comments and explicit checks to handleMessage and handleMUCMessage to make obvious where they differ.
Remove the "message" property from those GUI messages, since it should just use "text".
Move the login-fail handling to JS.
Remove useless sprintf calls from warning messages.
Inline some variables.
Add missing semicolons.