text chat

- implemented text chat between users
- by default hide debugging output for fetched stream
- fixed bug for joining newly created conference
- MacOS notifications for text chat messages
- UI for unread messages
NDN-CPP exceptions handling
UX improvements

- implemented conference list and user list mutual exclusive selections
- proper handling conference observers and chat instances upon changing
status (online/offline)
UX improvements

- added focus highlight for conversation info view
updated conference discovery and chronochat libraries. handling NFD shutdown at runtime
restarting face when NFD shut down
fixed crash when showing statistics window with no consumers
fixed crash when ending conversation
stability fixes
added crypto library to the bundle
updated Appcast.xml
'dev'->'master'