Import changes to tests/methcla_tests.hpp from pro/develop
Add beginnings of plugin C++ API
Refactor plugin registration in C++ API
C++ API: Add StaticSynthClass typedef
C++ API: Call Synth destructor in destroy callback
C++ API: Call Synth::activate from activate callback
Fix Methcla::Tests::rmsError for empty inputs
Add start frame option to sampler plugin