This is the first step in transitioning Piano to a full LibDSP backend.
For now, the delay effect is replaced with a (mostly identical)
implementation in LibDSP.
The new ProcessorParameterSlider attaches to a LibDSP::Processor's
range parameter (LibDSP::ProcessorRangeParameter) and changes it
automatically. It also has the ability to update an external GUI::Label.
This is used for the three delay parameters and it will become useful
for auto-generating UI for Processors.
