cylc gui: detect if a server is already running

[oliver-sanders]

As of #230 we have reincarnated the cylc gui command.

If a server is already running and the user runs the cylc gui command again, a second server will be started which is likely not what they wanted.

This is likely to be a bugbear for Cylc 7 users who may type cylc gui automatically.

We could detect when a server is running using a flag file (e.g. in the ~/.cylc/hub dir) and:

• Open a new browser tab pointing at the running server rather than starting a new one.
• Or send a message to the UI app telling it to change its focus to the selected workflow:
  • See cylc gui: add workflow ID argument #239

We should take a look at Jupyter Server to see if it has any related functionality.

Related to #238, #239

[oliver-sanders]

Closed by #370