Additional jack channels audio output

It would be nice to have a possiblity to add jack channels output in LMMS (as it exists in Hydrogen), this would allow for routing audio to external mixer (like non-mixer which has ambisonics) or effects (like rakarrack) to further expand LMMS capabilities and modularities. This would take the form of additional jack channels audio output. A typical implementation would for example consist in adding a checkbox labelled "jack send" on each mixer strip (off by default) that would activate two additional (stereo) jack channels output for the current channel mixer (each L/R channel would be prefixed with the related mixer strip label, those which are by default "FX 1", "FX 2",etc).

Start by locating LMMS's mixer-strip and JACK audio-output entry points, then compare the proposed behavior with Hydrogen's channel routing. Determine how optional stereo sends, strip labels, and the default-off checkbox should integrate with the existing mixer. Done means selected mixer strips expose correctly named L/R JACK outputs for routing to external mixers or effects.

Written by the indexing model from the issue text.