CHUI-151 FIXED (Implement conversation log)

A brief explanation of what have been implemented. More information can be found in comments.

1. Created conversation history viewer (llfloaterconversationpreview)
2. Created LLConversation and LLConversationLog classes which represent and hold data of conversations (llconversationlog)
3. Created LLConversationLogList and LLConversationLogListItem which are the visual representation of LLConversationLog and LLConversation respectively
4. Created LLFloaterConversationLog - which holds and displays LLConversationLogList