Target volume slide effect (Nxy)

[ipidev]

Implemented a target volume slide effect as Nxy, x = speed, y = target volume. The NSF drivers have been updated accordingly.

I designed it to be a volume-equivalent of the portamento effects Qxy/Rxy (which means I guess you also could consider Axy the volume-equivalent of 1xx/2xx). This means that the effect stops once the target volume is reached, or if the volume is set to a different value. The effect is also mutually exclusive with Axy as they both modify the channel volume.

I've tested both the tracker and NSF export for all channels that support the channel volume.

[nyanpasu64]

This is unrelated to whether this PR gets accepted or not, but personally I'm interested in adding duration-target effects for volume, pitch, and portamento, rather than slope-duration/target. Either this would be a separate effect from the existing ones, or replace the existing slope or slope-target effects in modules with a new setting enabled. Is that type of effect viable to implement in the NSF driver, and if so can you open a separate PR for that?

[ipidev]

There's no really good way to do that in 1) a way that fits into a single byte parameter, 2) is intuitive to use, and 3) works for consistently for all effects. Any solution would be really cumbersome to implement without rewriting large parts of the driver.