add methods to callback.js
automatically reconnect js client on disconnection
mixer-demo do not recreate UI on reconnection
NO-OP: indentation in message.js
make client JS reconnection optional
fix mixer-demo scrolling
minor JS client refactor
improve mixer-demo readability
"Always use Tabstops for block-indent (the code must be
formatted correctly with "[TAB] = N spaces" for any value of N).
Use space only for alignment." - https://ardour.org/styleguide.html
