Add a way to select `Thing`s from a `thing_slot`

The new thing_slot introduced in #194 should be ideal for plugins: by declaring a mapping slot with the right type, a Thing can access all instances of PluginThing on the server.

It's common to want to choose a plugin - e.g. set the current method used for background detection or scan planning in an OFM. Currently, this can be done with a string - but it would be lovely to have the available Thing names appear as enum options, so that UI code can automatically generate a drop-down selector.

This is not possible at the moment, because the TD is generated from static types, and said static types don't know what Things are available in the server.

I propose we allow the model (but not the value_type) of properties to depend on the host Thing. Doing so means that we could declare the property as a string, but include it in the TD as an enum. We could do run-time validation of the string, but by restricting it to the correct values in the schema, we make it much easier to generate a nice UI.

This is unlikely to make it for the next release, but maybe the next one...

Start by reading the thing_slot implementation introduced in #194 and the code that generates the TD from static types. Trace how a host Thing could supply available Thing names to the property model without changing value_type. Done means the TD exposes those names as enum options and runtime validation remains limited to valid selections.