add pregain + move postgain after sends

[felixroos]

the postgain control was a bit misleading, as it was still applied before the effect sends. this PR adds another gain stage called pregain be the gain stage before the effect sends (but after local effects). Additionally, if a value is set to postgain, another gain stage will be added at the very end. this should allow full wet effects in the future. The signal path is now:

source -> gain -> local fx -> pregain -> sends -> postgain

[daslyfe]

I wonder if it would be possible to evaluate the fx and chain them in the order they are written also, then you might not even need all three gain types.