Implement chatting during the game

[puzzlepaint]

• Upon pressing the Return key, a chat window should appear that allows entering chat messages.
• The existing ClientToServerMessage::Chat and ServerToClientMessage::ChatBroadcast network messages should be re-used to implement the distribution of the chat message.
• A text area needs to be implemented to display received chat messages. (Differing from the original game, it might be worth thinking about separating this from in-game messages to improve the visibility of chat messages. For example, the chat could be shown in the top-center part of the screen.)
• “Playername resigned.” type of messages could be shown in the same area.