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/component.cpp

@@ -19,10 +19,10 @@
 YaComponent::ConstructArgs::ConstructArgs() {}
 
 YaComponent::ConstructArgs::ConstructArgs(
-    Steinberg::IPtr<Steinberg::FUnknown> object) {
-    auto component = Steinberg::FUnknownPtr<Steinberg::Vst::IComponent>(object);
-
-    if (component) {
+    Steinberg::IPtr<Steinberg::FUnknown> object)
+    : supported(false) {
+    if (auto component =
+            Steinberg::FUnknownPtr<Steinberg::Vst::IComponent>(object)) {
         supported = true;
 
         // `IComponent::getControllerClassId`