Make YaPluginFactory abstract

And add separate implementations for the native plugin and the Wine
plugin host. This way we can easily allow the native host to do
callbacks without having to manage a load of lambdas.

meson.build

@@ -82,6 +82,7 @@ vst3_plugin_sources = [
   'src/common/plugins.cpp',
   'src/common/utils.cpp',
   'src/plugin/bridges/vst3.cpp',
+  'src/plugin/bridges/vst3-impls.cpp',
   'src/plugin/host-process.cpp',
   'src/plugin/utils.cpp',
   'src/plugin/vst3-plugin.cpp',
@@ -109,6 +110,7 @@ if with_vst3
     'src/common/logging/vst3.cpp',
     'src/common/serialization/vst3/plugin-factory.cpp',
     'src/wine-host/bridges/vst3.cpp',
+    'src/wine-host/bridges/vst3-impls.cpp',
   ]
 endif