Dynamic Compressor Effect

Hi all,

I've polished my Dynamic Compressor effect and think it is ready now
for inclusion.

Since my last mail (see
https://sourceforge.net/p/audacity/mailman/message/37109016/ for the
initial message), I mainly reworked the user interface accoring to your
feedback and fixed several bugs.

The effect is basically realtime capable but for now realtime preview
is disabled after some discussion with Steve because there are
currently some limitations in the RealtimeEffectManager which cause 
bad user experience due to high processing latency in the effect at
some settings.

Main probleme here is, that the RealtimeEffectManager gives the effect
a small block, e.g. 256 samples, and expects the same amount of samples
in return. So if an effect like my compressor has a high processing
latency, there can be an initial silence for over one minute which the
users will think is a bug. Any changes made to the effect settings will
only take effect after this long time as well.

A possible solution would be latency compensation in the
RealtimeEffectManager so that the effect can request the required
amount of samples in advance at the beginning or when the user changes
settings. The silence at the beginning can then be discarded so that
users do not notice it. But this is out of scope of the compressor
effect changes.

The lastest revision including CI builds can be found in my pull
request at https://github.com/audacity/audacity/pull/676.

The latest documentation for the effect is can be found at
https://alphamanual.audacityteam.org/man/Dynamic_Compressor.

I'm looking forward to further feedback and integration.

Max