This is an update to the surface ArdourFeedback class that is needed to support
the new event loop integration method.
The various session event callbacks cannot be queued in the surface event loop
because that would create a delay between the time such events are fired and
the time for writing to clients arrive, due to lws_service() blocking while
it waits to read. To solve this issue a helper AbstractUI is created for
catching events as soon as possible and issuing a call to lws_cancel_service().
See WebsocketsServer::glib_idle_callback()
Some distro repositories offer versions of libwebsockets that have not been
compiled with LWS_WITH_GLIB or LWS_WITH_EXTERNAL_POLL enabled. For such cases
a different event loop integration method is needed.
True for Ubuntu 20.04 as of Jun '21
"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
