Add an action that can set assignments of FX returns in mixes and LR

[jswalden]

In my poking through, refactoring, rewriting, module code, I just discovered that you can set the pan/balance of an FX return in a mix or LR -- but you can't control the actual assignment of an FX return in a mix or LR. The parameter base for FX return to mixes is 66 14, to LR is 60 3C (those are all hex) -- but there aren't any hits for 66 in the code at all.

It looks like controlling FX return -> mix assignment was omitted by accident. Either that or the relevant table wasn't present in earlier versions of the SQ MIDI protocol document than v3 (which seems to be what the module code was based upon, given it's included in the source code) and that omission got reflected into code.

[jswalden]

Fixed in dfbd617. No promises as to when that will be folded into an actual release.