Plugin for VST and LV2

[jpcima]

The core of the effect was engineered as library, so it can be reused for a plugin version.

What is to do:

• the entire GUI (but use existing Fltk GUI as the model)
• the messaging around the realtime core (take also from Fltk)

Since this plugin is simple enough, it's decent opportunity to try DPF as framework.

[jpcima]

There exists now a VST and LV2 version of the chorus in the master branch.
Tested in jalv, and Carla in both versions.
Functionality is complete but UI can still get improvement.

notify @trebmuh who asked for plugin version in IRC discussion