axis: Expose custom axis state as `_name:description`

Allow to expose custom state from from the control system to the UI.

A bunch of common states are provided by BLISS and Daiquiri.

But some controllers (it's a feature from BLISS) can create there own axis states.

As result sometimes Daiquiri can't expose the real known state of an axis.

Now the state exposed to the web API can contain the following strings

• Known states like READY (as before)
• _{name} a custom state
• _{name}:{description} a custom state with a description

This way we should support any states exposed by BLISS.

The BLISS mocked axes pprovides a custom PARKED state. It can be tested like that:

DEMO_SESSION [1]: srot.controller.custom_park(srot)
DEMO_SESSION [2]: srot.sync_hard()