Enable Lv2 CV ports by interpolating

### Enhancement Summary

Enable Lv2 CV ports. These are control ports with audio rate. LMMS currently does not support sending audio into a control port, but we can, for now, interpolate normal `Model` values, like in the [blop interpolator](https://gitlab.com/drobilla/blop-lv2/-/blob/master/src/interpolator.c).

#### Justification

It enables more effects.

Start by reviewing how LMMS handles normal Model values and LV2 ports, then compare the linked blop interpolator implementation. The work is complete when LV2 CV ports with audio rate can receive interpolated Model values and the intended effects are enabled.

Written by the indexing model from the issue text.