Inspector: add support for removing probes from the frontend and backend.

* Alter the backend so probes are cleared one at a time, rather than all at once.
* Add Probe.removeProbe and Probe.probeRemoved to the inspector protocol.
* Simplify how probe group colors are updated.
* Move the probe group editor gutter marker to ProbeGroupDetailsSection.
* Teach the probe group and manager to remove probes and groups as necessary.
* Change how probe group keys are persisted.