Create Vst3HostContextProxy from YaHostApplication

This is quite a huge refactor, but note everything is consistent (and
we're going to need one or two more of these `Vst3*Proxy` objects).
Right now nothing extends `IHostApplication`, but this way it will be
trivial to add support for more host context interfaces.

src/common/serialization/vst3/plugin/audio-processor.h

@@ -21,7 +21,6 @@
 
 #include "../../common.h"
 #include "../base.h"
-#include "../host-application.h"
 #include "../process-data.h"
 
 #pragma GCC diagnostic push