* Move the updater into another process.
  This removes the limitation that caused Tui to crash with active workflows.
  Closes cylc#3527
* Add multi-workflow capability.
  Closes cylc#3464
* Add visual regression testing framework.
  Closes cylc#3530

* Fixes an obscure issue where the applications main loop becomes
  suspended as the result of attempting to register an `on_press`
  callback which takes as an argument a Cylc client.
* Even though this callback is not actually called at this point,
  just passing the reference to it appears to be enough to cause
  thread deadlocking because Cylc clients are async based and
  not thread safe.

* Post-refactor clean up of dangling code.

* Add a log view to tui and add it to the context menu for workflows,
  tasks and jobs.
* Allow raikura sessions to see module-scoped fixtures when appropriate.
* Fix a bug (which only surfaced in tests for some reason) where a
  colour was specified as an empty string which urwid doesn't appear to
  like.

* Add `cylc show` support for tasks in Tui.