Multi Session fixes:
* send user chat message to all target user Sessions (and not only first one!)
* send user-change data to all User Sessions (and not only first one!)
* make Usermanager multi Session compatible (all sessions can now be accessed external!)

Server::Usermanager:
* improve design by little restructure of function position in cpp (to make it easy readable!)
* extend comments

Other:
* code Improving