Added call started/ended message to the chat window.

- Added function `friend_add_str_message` to simplify addition of
  messages
- Added message to the chat window on call start/end.
better function name
Merge branch 'mrshu-call-notifications'
desktop loopback audio input for windows
small fixes
fix issue
small fix
better behavior
split with utf8
blue links
small fix
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small fix
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oops
fix windows save issue
fix resuming broken file transfer
.
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X11: scale image in resized video window