Dynamic track calculation

[helgoboss]

See https://forum.cockos.com/showpost.php?p=2396429&postcount=697

Would be nice to let the user calculate the target track depending on the value of a certain ReaLearn parameter or MIDI source channel.

[helgoboss]

It would be great to be able to link more than just tracks/FX/parameter to parameter values. And it would be good to let the formula refer to the value which is set for the particular mapping, in order to make things like #204 possible in combination with #161.