Look up documents by name and type

[epatters]

UUIDs are perfectly unique but are also unwieldy and unfriendly. It should be possible to look up documents by a combination of

• type, e.g., "model" or "diagram,
• human-readable name

Fortunately, as of #369, the latter is now standardized across document types.

Implementing this involves:

• Add an RPC method the backend, backed by a suitable SQL query
• Exposing this through a nice entry widget in the frontend, with completion menu