ChatMessage.cs: Added a class which represents the chat messages.
Client.cs: Changed type of gui-attribute from Form to DefaultGui so you can use the default-GUI's methods. Added a method to send chat messages from the backend to the GUI.
Backend.cs: Added a queue which stores the chat messages. Added methods to get and set the chat messages in the queue.
IBackend.cs: Added the methods that werde necessary for Backend.cs.