keep disabled engines in lists

[pft]

A few engines, among which SooperLooper, PianoTeq, setBfree, Aeolus, mod-ui & pure data are not available after having been used once (only a single instance is allowed).

When they are unavailable, they do not appear anymore in the UI for choosing engines. This is confusing to the user (huh, was it in another place? Did my recent update just remove the engine? Am I going mad?).

I propose to still list them, but greyed out (and SELECT might skip them), accompanied with a hint as to why the engine is unavailable.

E.g.

SooperLooper (already running)

or

SooperLooper (there may be only one!)

or (too cool to handle):

SooperLooper (used by chain #)

[riban-bw]

I had a play with this, flagging unavailable engines similar to titles/dividers so that they have a different colour and are skipped in the list but it looked wrong. Instead of the engines looking disabled they looked like titles which was even more confusing. We could identify them in some way so that we could grey them out but before thinking about that it occurred to me that showing disabled / unavailable may be insufficient. It doesn't explain why they are greyed out. It may be sufficient to document this but users are already confused by them disappearing because they haven't seen it documented so is greying them out much better?