+str #16073 readPreferred for FlexiMerge

Java API coming up in a few minutes.

Refs #16073
Good tip, let's try if we can make it so... As this one actually has a longer story behind it:

I want user land to look like this: `readPreferred(preferred)(other, other2)`, so `+    def apply(preferred: InputHandle)(secondaries: InputHandle*)`, but this compiles down to the same signature as the generated apply from `case class X(preferred: InputHandle)(secondaries: InputHandle*)`. I'd rather not hand code hashcode/equals in the case class, so trying to figure out which type would be best to use for the field in the case class hm..

I'll dig more into it :-)

And it allows me to use my favourite scala "operator" as well, let's welcome the snail `match ...(..., secondaries @_*)`!