Added call started/ended message to the chat window.

This pull request adds a log message to the chat window on each user action Tox currently supports (cancell, invite, ring, start).

Here is a more descriptive list:
- Added function `friend_add_str_message` to simplify addition of
  messages
- Added message to the chat window on call start/end.
- Updated `callstatus` strings to look more like automatically generated
  messages

Note that a similar functionality could be implemented for file transfers.