1. Fixes in login/register modules:

CLIENT:
- if credentials are wrong, you don't have to disconnect
- fixing all unclear cases with buttons flags and labels content (status)
- GetServerMessageStatus(): includes all types of server operations - login, disconnect (info to all users), disconnect (info to disconnected), register
SERVER:
- Message transfered to ServerMessageType:
--- boolean flag in order to know if server accepted data or not
--- enum type of operation (login, disconnect (info to all users), disconnect (info to disconnected), register)
- DataBaseConnector:
--- Register() - now returns KeyValuePair<bool, string> (operation status, message)
--- Login() - now returns KeyValuePair<bool, string> (operation status, message)
- fix bug with ID label - it wasn't being cleared before setting the value (logged in more that once was showing also old id value)

2. Initial works with OperationWindow - creating Window in Client project + showing it after proper logged in (at this moment commented section)

NOTE: Disconnect in Server/Client side have to be changed a little bit after adding OptionWindow and others (at this moment it works only as a one window program)