Link commands to tooltip event

[fcollonval]

specifying lab commands to run when various events. This has been very
helpful on jupyterlab-starters, and would give JSON workflows a little more
power.

These last three kinda quack like "middleware," which could be registered
by plugins and opted into in a tour definition in the settings/command
API... As these would not be part of the official types, they'd need a new
place in the schema, e.g. "extras" or "transformers," e.g. {"markdown":
true, "xpath": true, "commands": true} at the simplest, though they might
need configuration...

Originally posted by @bollwyvl in #17 (comment)