Allow indirect IConnectionPoint connections

This is needed to support Ardour. These extra hops and serialization
steps will probably hurt performance, but outside of some huge hacks (to
connect the components directly anyways) there's not much else we can
do.

src/common/serialization/vst3/connection-point-proxy.h

@@ -38,40 +38,10 @@
  */
 class Vst3ConnectionPointProxy : public YaConnectionPoint {
    public:
-    /**
-     * These are the arguments for constructing a
-     * `Vst3ConnectionPointProxyImpl`.
-     */
-    struct ConstructArgs {
-        ConstructArgs();
-
-        /**
-         * Read from an existing object. We will try to mimic this object, so
-         * we'll support any interfaces this object also supports.
-         *
-         * This is not necessary in this case since the object has to support
-         * `IConnectionPoint`, but let's stay consistent with the overall style
-         * here.
-         */
-        ConstructArgs(Steinberg::IPtr<FUnknown> object,
-                      size_t owner_instance_id);
-
-        /**
-         * The unique instance identifier of the proxy object instance this
-         * connection proxy has been passed to and thus belongs to. This way we
-         * can refer to the correct 'actual' `IConnectionPoint` instance when
-         * the plugin calls `notify()` on this proxy object.
-         */
-        native_size_t owner_instance_id;
-
-        YaConnectionPoint::ConstructArgs connection_point_args;
-
-        template <typename S>
-        void serialize(S& s) {
-            s.value8b(owner_instance_id);
-            s.object(connection_point_args);
-        }
-    };
+    // We had to define this in `YaConnectionPoint` to work around circular
+    // includes
+    using ConstructArgs =
+        YaConnectionPoint::Vst3ConnectionPointProxyConstructArgs;
 
     /**
      * Instantiate this instance with arguments read from an actual