2nd ec for external flashes

[Marsu42]

Ml only implements ec for the first flash in the ui and scripting, but I usually shoot with multiple flashes in a:b-c mode where a a:b ratio or more often a 2nd ec for the c group is set. I'm pretty sure a lot of people use this to a low power fill flash from modeling flashes, so having it in the ml ui would be nice and appear less "Rebel"-like.