Dim endpoints with nothing plugged in

[jaehho]

On laptops with a UCM card, each HDMI/DisplayPort output and the headset jack's microphone is its own endpoint, and wiremix lists them exactly like the ones that can play, labeled "No route selected". On mine the Output Devices tab shows four sof-hda-dsp rows, and only one of them has anything connected.

WirePlumber never selects an unavailable route, so these endpoints have routes, all available: no, and no active route. That availability is already in State, so this uses it:

• An endpoint is unavailable when it has at least one route under the current profile and all of them are unavailable. Endpoints without routes, like Pro Audio nodes, never are.
• Unavailable endpoints are listed after the rest, and their title, target and volume are patched with a new node_unavailable theme style: DarkGray in default, DIM in nocolor, nothing in plain. They return to normal as soon as something is plugged in, since the view is rebuilt when the route changes.
• With no active route and a single route, the target shows that route, e.g. "HDMI / DisplayPort 1 Output (unavailable)", instead of "No route selected". This also tells otherwise identically named rows apart.

The second commit is independent and can be dropped: sinks and sources with node.virtual set (filter-chain, loopback and null sinks) show "Virtual" instead of "No route selected", a label they shared with unplugged devices.

Output Devices on 0.11.0, with headphones plugged in and a filter-chain sink:

  Speaker EQ                                   No route selected
  sof-hda-dsp                                  No route selected
  sof-hda-dsp                                  No route selected
  sof-hda-dsp                                  No route selected
◇ sof-hda-dsp                                         Headphones

With this branch (the last three rows are dimmed):

  Speaker EQ                                             Virtual
◇ sof-hda-dsp                                         Headphones
  sof-hda-dsp          HDMI / DisplayPort 3 Output (unavailable)
  sof-hda-dsp          HDMI / DisplayPort 2 Output (unavailable)
  sof-hda-dsp          HDMI / DisplayPort 1 Output (unavailable)

#58 covers endpoints with no routes at all, so the two don't overlap in behavior, though they touch neighboring lines in Node::from. If you'd rather keep serial order, the sorting is one line and easy to make optional.

Tested with new view tests (availability, ordering, target titles), the CI steps (fmt, clippy with -D warnings, rustdoc with -D warnings), and against a live PipeWire 1.6.8 / WirePlumber 0.5.17 graph.