crone/softcut: add / rename smoothing parameters.

[catfact]

in crone mixer client, smoothing is applied to routing levelsand pan (MixerClient::smoothLevels structure.) but there are no parameters to change the smoothing time constant. a couple people have asked about adding these.

similarly, there is smoothing on output level and pan for each softcut voice, also with fixed timing. more confusingly, there is a levelSlew parameter which only controls recoruld/erase levels.

it's easy to add these parameters.
is it a good idea?
should we rename existing softcut slew params?
should a single param control all mixer slew times?
&c

[lpgitadmin]

Slew on output level and on panning of a voice would be welcome from my point of view.

Name wise softcut.rate_slew_time makes total sense while currently softcut.level_slew_time is confusing.

[tehn]

smoothed level/pan on softcut voices makes for some fun modulation and sound design... initial tests were easy and satisfying