Add onClientChatboxLayoutChange event

[XJMLN]

Event: onClientChatboxLayoutChange Description: Triggered when the client changes any settings related to the chatbox layout.

Parameters: chatboxLayout: A table containing all CVars values related to the chatbox settings.

Usage Example:

addEventHandler("onClientChatboxLayoutChange", root,
    function(chatboxLayout)
          iprint(chatboxLayout)
    end
)

Resolves #696